MFC r291770: rc.subr: Check for running daemons before a custom start_cmd

MFC r291770: 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
Relnotes:	yes

git-svn-id: svn://svn.freebsd.org/base/stable/10@292450 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f