util: Blacklist some session-specific variables

Things like XDG_SESSION_ID should not be uploaded to the environment.
For example this is broken currently:

  1. SSH to your machine
  2. Log in to MATE Shell
  3. Log out
  4. Log in again
  5. Lock the screen
  6. Try to unlock

You can't, and this is because the XDG_SESSION_ID from the first session
(step 2) has leaked through to the second one (step 4), and so MATE
Shell is listening to the `logind` `UnlockSession` signal for the wrong
session. The SSH session established in step 1 serves to keep the
`systemd --user` instance alive, so that the state is not torn down
between logins.

Original patch for GNOME by Iain Lane <iainl@gnome.org>.

Patch ported over to MATE's session manager by Mike Gabriel
<sunweaver@debian.org>.

Fixes mate-desktop#271

mate-session/gsm-util.c

@@ -38,6 +38,13 @@
 
 static gchar *_saved_session_dir = NULL;
 
+static const char * const variable_blacklist[] = {
+    "XDG_SEAT",
+    "XDG_SESSION_ID",
+    "XDG_VTNR",
+    NULL
+};
+
 gchar **
 gsm_get_screen_locker_command (void)
 {
@@ -536,6 +543,9 @@ gsm_util_export_activation_environment (GError     **error)
                 const char *entry_name = entry_names[i];
                 const char *entry_value = g_getenv (entry_name);
 
+                if (g_strv_contains (variable_blacklist, entry_name))
+                    continue;
+
                 if (!g_utf8_validate (entry_name, -1, NULL))
                     continue;
 
@@ -603,8 +613,13 @@ gsm_util_export_user_environment (GError     **error)
                 return FALSE;
         }
 
+        entries = g_get_environ ();
+
+        for (; variable_blacklist[i] != NULL; i++)
+                entries = g_environ_unsetenv (entries, variable_blacklist[i]);
+
         g_variant_builder_init (&builder, G_VARIANT_TYPE ("as"));
-        for (entries = g_get_environ (); entries[i] != NULL; i++) {
+        for (i = 0; entries[i] != NULL; i++) {
                 const char *entry = entries[i];
 
                 if (!g_utf8_validate (entry, -1, NULL))