x11/pcdm: Fix rc script for FreeBSD

PCDM doesn't daemonize/fork, so we need to handle this by using
daemon(8). It seems this port has only been used/tested on TrueOS with
OpenRC, so it has been broken in FreeBSD ports for the last two years.

PR: 227034
Reported by: VulcanRider
Approved by: just fix it
MFH: 2019Q4
Sponsored by: vBSDcon 2019