fix: more issue #112 follow-ups (stanza-id disco gate, account sanitizer)

- xmpp: gate XEP-0359 stanza-id dedup on disco urn:xmpp:sid:0. Before
  honoring <stanza-id by='…'/> for archive_id dedup, check that the entity
  in `by` (or its domain — that's what profanity disco's on connect)
  announces the namespace; fall back to no-dedup when caps are unknown,
  matching the XEP §6 disco-gate (#1).
- accounts: narrow group-name sanitizer to only the characters GKeyFile
  actually forbids in group headers (`[`, `]`, `\n`, `\r`); `=` and `#` are
  valid inside a header and stay (#2).
- functional tests cover both branches of the disco gate — replayed
  stanza-id is flagged as duplicate when sid:0 is announced, and not
  flagged when it isn't.

Refs #112

tests/functionaltests/functionaltests.c

@@ -253,6 +253,10 @@ main(int argc, char* argv[])
         PROF_FUNC_TEST(message_receive_console),
         PROF_FUNC_TEST(message_receive_chatwin),
 
+        /* XEP-0359 disco gate for stanza-id trust */
+        PROF_FUNC_TEST(stanza_id_dedup_fires_when_server_announces_sid0),
+        PROF_FUNC_TEST(stanza_id_not_trusted_when_server_does_not_announce_sid0),
+
 #ifdef HAVE_SQLITE
         /* MUC (groupchat) database — export/import of type="muc" messages */
         PROF_FUNC_TEST(muc_export_sqlite_to_flatfile),