Properly daemonize the mail_room process

The old invocation only worked by accident because we have a '&'
somewhere in the init script for expediency. When ran from a terminal,
the mail_room daemon process ended up in the session of the terminal.
This commit adds a small wrapper that tries to do the textbook
daemonization steps (double fork, setsid etc.) while also taking
care that the pidfile is written before the 'start' process exits.