Introduce the 'usertemp' filesystem type

In the context of Git for Windows, the MSys2 root lives inside
%PROGRAMFILES% which is typically write-protected for mere mortals' user
accounts. In particular for /tmp/, this is a problem, because many
scripts assume that writing to /tmp/ is fair game.

MSys1 used to mount /tmp/ automatically, pointing to the current user's
temporary directory.

Let's recreate that functionality in a slightly different way: we now
interpret the filesystem type "usertemp" in the /etc/fstab file. To make
/tmp/ point to the temporary directory of the current user, as per the
%TMP% environment variable, just add this line to /etc/fstab:

	none /tmp usertemp binary,posix=0 0 0

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

winsup/cygwin/mount.cc

@@ -1148,6 +1148,8 @@ mount_info::from_fstab_line (char *line, bool user)
   unsigned mount_flags = MOUNT_SYSTEM | MOUNT_BINARY;
   if (!strcmp (fs_type, "cygdrive"))
     mount_flags |= MOUNT_NOPOSIX;
+  if (!strcmp (fs_type, "usertemp"))
+    mount_flags |= MOUNT_IMMUTABLE;
   if (!fstab_read_flags (&c, mount_flags, false))
     return true;
   if (mount_flags & MOUNT_BIND)
@@ -1172,6 +1174,21 @@ mount_info::from_fstab_line (char *line, bool user)
       slashify (posix_path, cygdrive, 1);
       cygdrive_len = strlen (cygdrive);
     }
+  else if (!strcmp (fs_type, "usertemp"))
+    {
+      WCHAR tmp[MAX_PATH];
+
+      if (GetEnvironmentVariableW (L"TEMP", tmp, sizeof(tmp)) && *tmp)
+	{
+          DWORD len;
+          char mb_tmp[len = sys_wcstombs (NULL, 0, tmp)];
+          sys_wcstombs (mb_tmp, len, tmp);
+
+	  int res = mount_table->add_item (mb_tmp, posix_path, mount_flags);
+	  if (res && get_errno () == EMFILE)
+	    return false;
+	}
+    }
   else
     {
       int res = mount_table->add_item (native_path, posix_path, mount_flags);