Mailling loop detection

[John Peacock]

Though a couple of self-inflicted Denial of Service attacks on my mail 
servers, I've discovered that qpsmtpd doesn't implement any mailing loop 
detection internally.  Now that I have fixed my misconfiguration (alias 
domains not completely set up), I thought I'd add this to qpsmtpd so it 
*can't* happen again.

The only question I have is whether this should be a plugin (since it 
might be computationally significant) or if it should be part of the 
core code...

Thoughts???
John