rc.subr: Check for running daemons before a custom start_cmd is executed.

Currently rc scripts implementing their own start_cmd do not enjoy the
benefits of rc.subr's own check for rc_pid.

This leads to around a third of ports with such a start_cmd not to check for
the process at all and two thirds of ports to re-implement this check
(sometimes wrongly).

This patch moves the check for rc_pid to before ${rc_arg}_cmd is executed.

Submitted by: Dirk Engling
Reviewed by: feld
MFC after: 1 week
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D4156