nss-systemd: synthesize NSS shadow/gshadow records from userdb, as well

This ensures we not only synthesize regular paswd/group records of
userdb records, but shadow records as well. This should make sure that
userdb can be used as comprehensive superset of the classic
passwd/group/shadow/gshadow functionality.

factory/etc/nsswitch.conf

@@ -2,7 +2,8 @@
 
 passwd:         compat systemd
 group:          compat [SUCCESS=merge] systemd
-shadow:         compat
+shadow:         compat systemd
+gshadow:        files systemd
 
 hosts:          mymachines resolve [!UNAVAIL=return] files myhostname dns
 networks:       files

man/nss-myhostname.xml

@@ -91,7 +91,9 @@
     <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
 <programlisting>passwd:         compat systemd
 group:          compat [SUCCESS=merge] systemd
-shadow:         compat
+shadow:         compat systemd
+gshadow:        files systemd
+
 
 # Either (untrusted network, see above):
 hosts:          mymachines resolve [!UNAVAIL=return] files <command>myhostname</command> dns

man/nss-mymachines.xml

@@ -57,7 +57,8 @@
     <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
     <programlisting>passwd:         compat systemd
 group:          compat [SUCCESS=merge] systemd
-shadow:         compat
+shadow:         compat systemd
+gshadow:        files systemd
 
 hosts:          <command>mymachines</command> resolve [!UNAVAIL=return] files myhostname dns
 networks:       files

man/nss-resolve.xml

@@ -63,7 +63,8 @@
     <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
 <programlisting>passwd:         compat systemd
 group:          compat [SUCCESS=merge] systemd
-shadow:         compat
+shadow:         compat systemd
+gshadow:        files systemd
 
 hosts:          mymachines <command>resolve [!UNAVAIL=return]</command> files myhostname dns
 networks:       files

man/nss-systemd.xml

@@ -47,11 +47,13 @@
     for resolving users and groups, but also works without the service running.</para>
 
     <para>To activate the NSS module, add <literal>systemd</literal> to the lines starting with
-    <literal>passwd:</literal> and <literal>group:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
+    <literal>passwd:</literal>, <literal>group:</literal>, <literal>shadow:</literal> and
+    <literal>gshadow:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>
 
     <para>It is recommended to place <literal>systemd</literal> after the <literal>files</literal> or
     <literal>compat</literal> entry of the <filename>/etc/nsswitch.conf</filename> lines so that
-    <filename>/etc/passwd</filename> and <filename>/etc/group</filename> based mappings take precedence.</para>
+    <filename>/etc/passwd</filename>, <filename>/etc/group</filename>, <filename>/etc/shadow</filename> and
+    <filename>/etc/gshadow</filename> based mappings take precedence.</para>
   </refsect1>
 
   <refsect1>
@@ -63,7 +65,8 @@
     <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
     <programlisting>passwd:         compat <command>systemd</command>
 group:          compat [SUCCESS=merge] <command>systemd</command>
-shadow:         compat
+shadow:         compat <command>systemd</command>
+gshadow:        files <command>systemd</command>
 
 hosts:          mymachines resolve [!UNAVAIL=return] files myhostname dns
 networks:       files

src/basic/nss-util.h

@@ -127,6 +127,20 @@ enum nss_status _nss_##module##_getpwuid_r(             \
                 char *buffer, size_t buflen,            \
                 int *errnop) _public_
 
+#define NSS_GETSP_PROTOTYPES(module)                    \
+enum nss_status _nss_##module##_getspnam_r(             \
+                const char *name,                       \
+                struct spwd *spwd,                      \
+                char *buffer, size_t buflen,            \
+                int *errnop) _public_
+
+#define NSS_GETSG_PROTOTYPES(module)                    \
+enum nss_status _nss_##module##_getsgnam_r(             \
+                const char *name,                       \
+                struct sgrp *sgrp,                      \
+                char *buffer, size_t buflen,            \
+                int *errnop) _public_
+
 #define NSS_GETGR_PROTOTYPES(module)                    \
 enum nss_status _nss_##module##_getgrnam_r(             \
                 const char *name,                       \
@@ -150,6 +164,17 @@ enum nss_status _nss_##module##_getpwent_r(             \
                 size_t buflen,                          \
                 int *errnop) _public_;
 
+#define NSS_SPENT_PROTOTYPES(module)                    \
+enum nss_status _nss_##module##_endspent(               \
+                void) _public_;                         \
+enum nss_status _nss_##module##_setspent(               \
+                int stayopen) _public_;                 \
+enum nss_status _nss_##module##_getspent_r(             \
+                struct spwd *spwd,                      \
+                char *buffer,                           \
+                size_t buflen,                          \
+                int *errnop) _public_;
+
 #define NSS_GRENT_PROTOTYPES(module)                    \
 enum nss_status _nss_##module##_endgrent(               \
                 void) _public_;                         \
@@ -161,6 +186,17 @@ enum nss_status _nss_##module##_getgrent_r(             \
                 size_t buflen,                          \
                 int *errnop) _public_;
 
+#define NSS_SGENT_PROTOTYPES(module)                    \
+enum nss_status _nss_##module##_endsgent(               \
+                void) _public_;                         \
+enum nss_status _nss_##module##_setsgent(               \
+                int stayopen) _public_;                 \
+enum nss_status _nss_##module##_getsgent_r(             \
+                struct sgrp *sgrp,                      \
+                char *buffer,                           \
+                size_t buflen,                          \
+                int *errnop) _public_;
+
 #define NSS_INITGROUPS_PROTOTYPE(module)                \
 enum nss_status _nss_##module##_initgroups_dyn(         \
                 const char *user,                       \