Add CLI flags to make ratmand easier to integrate

Currently ratmand runs forever. We should add some additional flags for --daemonize and --pidfile, etc

• --daemonize should fork ratmand into the background/ detach it from the current stdout/stderr/tty
• --pidfile should take an argument to a file which the process PID is written into (or new process PID after forking of combined with --daemonize)

This is mainly to make integration in NetBSD/ pkgsrc nicer.