Reduce "backscatter" bounce risk by setting local_recipient_maps

Leaving the local_recipient_maps setting empty has a risk which is
described in the Postfix manual:

   "[...] That is, an empty value. With this setting, the Postfix SMTP
    server will not reject mail with "User unknown in local recipient
    table". Don't do this on systems that receive mail directly from
    the Internet. With today's worms and viruses, Postfix will become a
    backscatter source: it accepts mail for non-existent recipients and
    then tries to return that mail as "undeliverable" to the often
    forged sender address."

This commit changes the local_recipient_maps setting to only accept
(and potentially bounce) emails where the local part is known (one
that we've mentioned in general.yml) or to a Unix user that exists.

Fixes #1166
Rename /etc/postfix/regexp to /etc/postfix/transports

This seems slightly less confusing, since both this file
and /etc/postfix/recipients contain regexp patterns.  This
also switches to replacing any transport_maps file that was
in use and overwrites the existing file rather than modifying
it.