Fix memory issues at shutdown

* Fix double-free of accounts menu widgets.
On shutdown, these are part of the menu, and are already destroyed, so
we can't destroy them again as part of finalize, which happens in the
hash table removal.

* Silence some warnings at shutdown.
If the IO operation is cancelled, then it doesn't need to be printed or
considered an error.

* Add config.h back to jabber.h.
This header checks config values (e.g., `HAVE_CYRUS_SASL`), so it should
include config, or else it falls to C sources to do it. And if they
don't, then they end up with incorrect struct offsets, which is causing
weird memory errors as stuff is put in the wrong place.

Testing Done:
valgrind with an xmpp account enabled.

Reviewed at https://reviews.imfreedom.org/r/51/