rc.subr: use _pidcmd to determine pid for protect

This is a more reliable method that accounts for existing pidfiles,
procname and interpreter settings.

Current method of obtaining the pid for oomprotect="YES"|"ALL" processes
in certain cases fails to find a unique pid.

One such case are rc.d scripts defining command as:
command="daemon"

which results in all processes started via daemon being selected and
passed to protect(1) which fails and prints usage:

$ /etc/rc.d/exampled restart
Stopping exampled.
Starting exampled.
usage: protect [-i] command

protect [-cdi] -g pgrp | -p pid

Running the same with -x reveals what happens:

+ pid='3051 4268 4390 4421 4427 4470 4588 4733 4740 4870 4949 4954 4979
5835 5866 55487 55583 56525 57643 57789 57882 58072 58167 99419'
+ /usr/bin/protect -p 3051 4268 4390 4421 4427 4470 4588 4733 4740 4870
4949 4954 4979 5835 5866 55487 55583 56525 57643 57789 57882 58072 58167
99419
usage: protect [-i] command

protect [-cdi] -g pgrp | -p pid

We have a more reliable way of obtaining pid already defined in rc.subr
and available when protect(1) needs it. We can simply eval $_pidcmd
which also invokes check_process but properly accounts for existing
pidfile, procname and interpreter settings.

With the change the pidfile is properly obtained.

Submitted by: Adam Wolk <a.wolk at fudosecurity.com>
Sponsored by: Fudo Security
Differential Revision: https://reviews.freebsd.org/D30367