Patch for default from_domain (via server variable)

My users unfortunately would be sending email as username@localhost for a good 10 years before they read my email saying "Set up your identity so you don't look like an idiot".

Either way I developed a small patch that creates the global config variable "from_domain" and is set to the servers variable MAILDOMAIN as in this example:

<VirtualHost *> .. SetEnv MAILDOMAIN mydomain.com ..

</VirtualHost>

this modifies only the email portion of the identity creation functions, if no MAILDOMAIN it will revert to the default behaviour.

Is this just a fixup in what would be a large patchset for better vhost setups? hopefully.