Improve RTG port

Summary:
Add 'rtg' user and group in UIDs/GIDs.
Use daemon(8) to daemonize rtgpoll.
Add prestart commands to set correct permissions for RTG's files.

Reviewers: swills, allanjude, xmj, andrew.fengler_scaleengine.com, Contributor Reviewers (ports), matthew

Reviewed By: Contributor Reviewers (ports), matthew

Subscribers: matthew, mat

Differential Revision: https://reviews.freebsd.org/D7486