glocalfile: Allow trashing only for locations supported by trash:///

New bugs appears periodically in nautilus/gvfs/glib components that not
all trashed files are shown in trash:///. It used to be problem mostly
for "bind mounts" and btrfs subvolumes only. Currently, it is also
problem for nfs, cifs and other filesystems, which have been recently
added by commmit 0d69462f on the list of system internal filesystems.

This happens because the trash backend doesn't monitor files on system
internal mounts. Such behavior is not against the trash-spec, however,
we should be consistent within GNOME.

This behavior has the nice side-effect that it solves issues with hangs
on network filesystems: https://gitlab.gnome.org/GNOME/glib/issues/605,
because those are currently on the system internal filesystem list.

https://gitlab.gnome.org/GNOME/glib/issues/251

gio/glocalfile.c

@@ -1930,6 +1930,7 @@ g_local_file_trash (GFile         *file,
     {
       uid_t uid;
       char uid_str[32];
+      GUnixMountEntry *mount;
 
       uid = geteuid ();
       g_snprintf (uid_str, sizeof (uid_str), "%lu", (unsigned long)uid);
@@ -1942,7 +1943,21 @@ g_local_file_trash (GFile         *file,
                           file, G_IO_ERROR_NOT_SUPPORTED);
 	  return FALSE;
 	}
-      
+
+      mount = g_unix_mount_at (topdir, NULL);
+      if (mount == NULL || g_unix_mount_is_system_internal (mount))
+        {
+          g_set_error (error, G_IO_ERROR, G_IO_ERROR_NOT_SUPPORTED,
+                       _("Trashing on system internal mounts is not supported"));
+
+          g_clear_pointer (&mount, g_unix_mount_free);
+          g_free (topdir);
+
+          return FALSE;
+        }
+
+      g_clear_pointer (&mount, g_unix_mount_free);
+
       /* Try looking for global trash dir $topdir/.Trash/$uid */
       globaldir = g_build_filename (topdir, ".Trash", NULL);
       if (g_lstat (globaldir, &global_stat) == 0 &&