Use imcb_buddy_nick_hint to suggest jabber nicknames

Seting default values to nick_format is no longer needed, so remove that
from the subprotocol descriptions.

And since mention_name is suggested as nickname but not as full name,
whois now shows the correct full name for hipchat users.
Fix reconnects when the server reports a different username

Added the concept of "account->user_internal" and account_is_handle() to
use this one from groupchats.

Also adapt jabber_set_me to set this internal username, not the main
one, which broke reconnections.