pop3d virtual domains

[Jakob 'sparky' Kaivo]

I just committed support for virtual domains in the pop3d. Take a look
at it, it will probably need a bit more testing. It should be fairly
easy to apply also to imap4d. Basically, it works this way:

1) Configure with --enable-virtual-domains
2) For each domain, there is a password file in /etc/domains named for
   the domain (eg. domain ndn.net has password file
   /etc/domains/ndn.net) which is structered just like /etc/passwd
3) When a user logs in, it first tries other available authentication
   methods, and only does virtual as a last resort. If the username
   has in it a !, :, or @, the left part is the username and right
   part is the domain to use.
4) Authenticate user vs. /etc/domains/<domain>. If successful, we
   append "/INBOX" to the home directory field from
   /etc/domains/<domain> and open it (this way, we can add support in
   imap4d and will get the same INBOX).

For those who don't know, I am leaving on Monday for Basic Training in
the US Army, so I'll be offline for a few months. Naturally, I feel it
necessary to go about messing lots of things up and playing with new
code right before I leave, so I'm not around for people to get mad
at. ;) Comments are welcome until then, though.

-- sparky