[Users] default domain

[Jeremy Schneider]

Just a minor thing - I noticed recently that claws picks up its default
domain from my laptop. It's easy enough to set the domain in the
advanced setting for the account.

But the thought did occur to me that if an IMAP login name includes a
domain name, then it might be logical to use that as a default over the
local laptop settings. I'm using IMAP with gmail, so my actual login
name matches my email address - schneider at ardentperf.com - and it's not
hard to guess that I'm going to set my domain to ardentperf.com for
emails on this account.  :)

If there was interest, it should be a pretty simple pattern matching
exercise to check for "@" in the IMAP account login string.

-Jeremy