fix: allow adding own JID to roster

Enable adding oneself to the roster.

Self subscriptions are implicit according to XEP-0060 / XEP-0163.
So we adapt the /sub command to handle this gracefully by just printing
an informative message.

Profanitys logic didn't handle own presence/when adding to roster
correctly. This got fixed now.

Fixes: https://github.com/profanity-im/profanity/issues/2084

Signed-off-by: Michael Vetter <jubalh@iodoru.org>

tests/unittests/xmpp/stub_xmpp.c

@@ -610,3 +610,9 @@ void
 blocked_ac_reset(void)
 {
 }
+
+GList*
+connection_get_available_resources(void)
+{
+    return NULL;
+}