lib: use 'localhost' rather than fqdn for default mail address.

lib: use 'localhost' rather than fqdn for default mail address.

As discussed in the thread starting at [1], the fully qualified domain
name is a bit tricky to get reproducibly, might reveal information
people prefer to keep private, and somewhat unlikely to provide
reliable mail routing.

The new approach of $current_username@localhost is better for the
first two considerations, and probably at least as good as a test mail
address.

[1]: id:87sfyibqhj.fsf@tethera.net