Add debug option for verbose logging

This only does stuff to the dovecot service for now, but I thought it might be useful to have one option that enables verbose logging for the various services that nixos-mailserver uses.

There's actually another dovecot option that I find useful while setting up a mailserver for the first time, auth_debug_passwords = yes but I have deliberately excluded that because it's kind of unsafe. If someone really wants that it' possible to add it to services.dovecot2.extraConfig.