databases/rrdtool: Don't hardcode in start script

Upate the start script of the rrdtool component rrdcached to not hardcode
things like group, socket and pid.
These can now be overridden in /etc/rc.conf with rrdcached_group,
rrdcached_address and rrdcached_pid, respectively.
The defaults are still the same.

PR: 246122
Submitted by: Dries Michiels
MFH: 2020Q3