nss-resolve: return NOTFOUND instead of UNAVAIL on resolution errors

It needs to be possible to tell apart "the nss-resolve module does not exist"
(which can happen when running foreign-architecture programs) from "the queried
DNS name failed DNSSEC validation" or other errors. So return NOTFOUND for these
cases too, and only keep UNAVAIL for the cases where we cannot handle the given
address family.

This makes it possible to configure a fallback to "dns" without breaking
DNSSEC, with "resolve [!UNAVAIL=return] dns". Add this to the manpage.

This does not change behaviour if resolved is not running, as that already
falls back to the "dns" glibc module.

Fixes systemd#4157

man/nss-resolve.xml

@@ -95,6 +95,12 @@ rpc:            db files
 
 netgroup:       nis</programlisting>
 
+    <para>If the <command>dns</command> module needs to be kept for cases where <command>nss-resolve</command> is not
+    available (like running foreign-architecture programs), configure <literal>hosts</literal> in
+    <filename>/etc/nsswitch.conf</filename> like this instead:</para>
+
+<programlisting>
+hosts:          files mymachines myhostname <command>resolve [!UNAVAIL=return]</command> dns</programlisting>
   </refsect1>
 
   <refsect1>

src/nss-resolve/nss-resolve.c

@@ -279,9 +279,12 @@ enum nss_status _nss_resolve_gethostbyname4_r(
         }
 
 fail:
+        /* When we arrive here, resolved runs and has answered (fallback to
+         * "dns" is handled earlier). So we have a definitive "no" answer and
+         * should not fall back to subsequent NSS modules via "UNAVAIL". */
         *errnop = -r;
         *h_errnop = NO_RECOVERY;
-        return NSS_STATUS_UNAVAIL;
+        return NSS_STATUS_NOTFOUND;
 }
 
 enum nss_status _nss_resolve_gethostbyname3_r(
@@ -476,7 +479,7 @@ enum nss_status _nss_resolve_gethostbyname3_r(
 fail:
         *errnop = -r;
         *h_errnop = NO_RECOVERY;
-        return NSS_STATUS_UNAVAIL;
+        return NSS_STATUS_NOTFOUND;
 }
 
 enum nss_status _nss_resolve_gethostbyaddr2_r(
@@ -666,7 +669,7 @@ enum nss_status _nss_resolve_gethostbyaddr2_r(
 fail:
         *errnop = -r;
         *h_errnop = NO_RECOVERY;
-        return NSS_STATUS_UNAVAIL;
+        return NSS_STATUS_NOTFOUND;
 }
 
 NSS_GETHOSTBYNAME_FALLBACKS(resolve);