Allow overriding the home directory via the HOME variable

In Git for Windows, it is a well-established technique to use the
`$HOME` variable to define where the current user's home directory is,
falling back to `$HOMEDRIVE$HOMEPATH` and `$USERPROFILE`.

This strategy is particular important when Cygwin, or command-line
programs depending on the HOME variable, cannot cope with the Windows'
idea of the user's home directory e.g. when it is set to a hidden
directory via an UNC path (\\share\some\hidden\folder$).

Of course this strategy needs to be opt-in. For that reason, this
strategy is activated via the `env` keyword in the `db_home` line in
`/etc/nsswitch.conf`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

winsup/cygwin/cygheap.h

@@ -403,7 +403,8 @@ class cygheap_pwdgrp
     NSS_SCHEME_UNIX,
     NSS_SCHEME_DESC,
     NSS_SCHEME_PATH,
-    NSS_SCHEME_FREEATTR
+    NSS_SCHEME_FREEATTR,
+    NSS_SCHEME_ENV
   };
   struct nss_scheme_t {
     nss_scheme_method	method;

winsup/cygwin/uinfo.cc

@@ -794,6 +794,8 @@ cygheap_pwdgrp::nss_init_line (const char *line)
 		    scheme[idx].method = NSS_SCHEME_UNIX;
 		  else if (NSS_CMP ("desc"))
 		    scheme[idx].method = NSS_SCHEME_DESC;
+		  else if (NSS_CMP ("env"))
+		    scheme[idx].method = NSS_SCHEME_ENV;
 		  else if (NSS_NCMP ("/"))
 		    {
 		      const char *e = c + strcspn (c, " \t");
@@ -982,6 +984,40 @@ fetch_from_path (cyg_ldap *pldap, PUSER_INFO_3 ui, cygpsid &sid, PCWSTR str,
   return ret;
 }
 
+static size_t
+fetch_env(LPCWSTR key, char *buf, size_t size)
+{
+  WCHAR wbuf[32767];
+  DWORD max = sizeof wbuf / sizeof *wbuf;
+  DWORD len = GetEnvironmentVariableW (key, wbuf, max);
+
+  if (!len || len >= max)
+    return 0;
+
+  len = sys_wcstombs (buf, size, wbuf, len);
+  return len && len < size ? len : 0;
+}
+
+static char *
+fetch_home_env (void)
+{
+  char home[32767];
+  size_t max = sizeof home / sizeof *home, len;
+
+  if (fetch_env (L"HOME", home, max)
+      || ((len = fetch_env (L"HOMEDRIVE", home, max))
+        && fetch_env (L"HOMEPATH", home + len, max - len))
+      || fetch_env (L"USERPROFILE", home, max))
+    {
+      tmp_pathbuf tp;
+      cygwin_conv_path (CCP_WIN_A_TO_POSIX | CCP_ABSOLUTE,
+	  home, tp.c_get(), NT_MAX_PATH);
+      return strdup(tp.c_get());
+    }
+
+  return NULL;
+}
+
 char *
 cygheap_pwdgrp::get_home (cyg_ldap *pldap, cygpsid &sid, PCWSTR dom,
 			  PCWSTR dnsdomain, PCWSTR name, bool full_qualified)
@@ -1041,6 +1077,10 @@ cygheap_pwdgrp::get_home (cyg_ldap *pldap, cygpsid &sid, PCWSTR dom,
 		}
 	    }
 	  break;
+	case NSS_SCHEME_ENV:
+	  if (RtlEqualSid (sid, cygheap->user.sid ()))
+	    home = fetch_home_env ();
+	  break;
 	}
     }
   return home;
@@ -1073,6 +1113,10 @@ cygheap_pwdgrp::get_home (PUSER_INFO_3 ui, cygpsid &sid, PCWSTR dom,
 	  home = fetch_from_path (NULL, ui, sid, home_scheme[idx].attrib,
 				  dom, NULL, name, full_qualified);
 	  break;
+	case NSS_SCHEME_ENV:
+	  if (RtlEqualSid (sid, cygheap->user.sid ()))
+	    home = fetch_home_env ();
+	  break;
 	}
     }
   return home;
@@ -1092,6 +1136,7 @@ cygheap_pwdgrp::get_shell (cyg_ldap *pldap, cygpsid &sid, PCWSTR dom,
 	case NSS_SCHEME_FALLBACK:
 	  return NULL;
 	case NSS_SCHEME_WINDOWS:
+	case NSS_SCHEME_ENV:
 	  break;
 	case NSS_SCHEME_CYGWIN:
 	  if (pldap->fetch_ad_account (sid, false, dnsdomain))
@@ -1156,6 +1201,7 @@ cygheap_pwdgrp::get_shell (PUSER_INFO_3 ui, cygpsid &sid, PCWSTR dom,
 	case NSS_SCHEME_CYGWIN:
 	case NSS_SCHEME_UNIX:
 	case NSS_SCHEME_FREEATTR:
+	case NSS_SCHEME_ENV:
 	  break;
 	case NSS_SCHEME_DESC:
 	  if (ui)
@@ -1237,6 +1283,8 @@ cygheap_pwdgrp::get_gecos (cyg_ldap *pldap, cygpsid &sid, PCWSTR dom,
 		sys_wcstombs_alloc (&gecos, HEAP_NOTHEAP, val);
 	    }
 	  break;
+	case NSS_SCHEME_ENV:
+	  break;
 	}
     }
   if (gecos)
@@ -1263,6 +1311,7 @@ cygheap_pwdgrp::get_gecos (PUSER_INFO_3 ui, cygpsid &sid, PCWSTR dom,
 	case NSS_SCHEME_CYGWIN:
 	case NSS_SCHEME_UNIX:
 	case NSS_SCHEME_FREEATTR:
+	case NSS_SCHEME_ENV:
 	  break;
 	case NSS_SCHEME_DESC:
 	  if (ui)