Backport the LOGINDISABLED check to stable. Unfortunately, in stable the
whole connect/login process is still integrated in one big function, so
I had to devise some workaround as to still be able to detect LOGINDISABLED
but not disturb too much code.

This check is required by RFC since that states that a client MUST NOT send
LOGIN when LOGINDISABLED is advertised. It also eases debugging (doesn't just
return User/pw incorrect anymore), esp of UW-IMAP.