Implemented /group command with no args (list groups)

src/xmpp/roster.c

@@ -378,6 +378,12 @@ roster_get_contacts(void)
     return result;
 }
 
+GSList *
+roster_get_groups(void)
+{
+    return autocomplete_get_list(groups_ac);
+}
+
 GSList *
 roster_get_group(const char * const group)
 {