Add MTA for Mailman and Hydra

[aszlig]

This is useful for sending build failure notifications and for serving public malinglists in conjunction with Mailman.

Things left to do in no particular order:

• Rewrite the type for config so that whenever something like config.alias_maps.hash is specified, the corresponding map file is automatically created.
• Run each Postfix service as a different user.
• Provide a sane default configuration.
• Add module assertions for impossible service configuration combinations.
• Improve NixOS VM test.
• Handle processLimit.
• Set ambient capabilities for Postfix services.
• Move sockets to /run.
• Implement chroot and namespacing.
• Add a postfix.target to better control all Postfix service units at once.
• Implement waking up Postfix services.
• Strip down postmap to just map file creation.
• Properly set permissions on sockt files.
• Get rid of postdrop group.
• Add an option for Postfix service description.
• Implement custom merge function for the config option so that scalars is merged using lists.
• Take care of helper programs like sendmail or postqueue.
• Perform extra validation of the config file during build time.
• Log to stderr instead of syslog.
• Adjust throttling of service restarts.
• Implement high level abstractions (like eg. cfg.maps or cfg.restrictions).

[aszlig]

Cc: @Profpatsch, @richi235, @salomonderossi, @waaaaargh, @moba