mail/mailman3: Update rc.d script

Fix the exit code of "service mailman status". Mailman's own status
command exits 0 when it is not running and 1 when it is, which is the
inverse of the rc.subr convention. Wrap it so that a running master
yields 0, as service(8) consumers such as Ansible expect.

Also add a start_precmd that detects an already running master via the
pidfile, so that "service mailman start" no longer dumps a full click
usage block on a second invocation.

PR: 292404
Sponsored by: Netzkommune GmbH