Update FreeBSD RC script

Update FreeBSD RC script

- removed /usr/sbin/daemon's option "-t" (first appeared in FreeBSD 11, leading to an error in previous versions; since the [t]itle defaults to the daemonized invocation anyways, this shouldn't make a difference for FreeBSD 11+)
- removed /usr/sbin/daemon's option "-u" and the respective variable in the script (ympd has to be started as root to be able to bind to port 80)
- allow for additional arguments via ympd_flags="" in rc.conf (which would again allow to drop privileges after binding to the port)