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

libexec/rc/rc.subr

@@ -1272,13 +1272,13 @@ $command $rc_flags $command_args"
 			# We cannot use protect(1) inside jails.
 			if [ -n "$_oomprotect" ] && [ -f "${PROTECT}" ] &&
 			    [ "$(sysctl -n security.jail.jailed)" -eq 0 ]; then
-				pid=$(check_process $command)
+				[ -z "${rc_pid}" ] && eval $_pidcmd
 				case $_oomprotect in
 				[Aa][Ll][Ll])
-					${PROTECT} -i -p ${pid}
+					${PROTECT} -i -p ${rc_pid}
 					;;
 				[Yy][Ee][Ss])
-					${PROTECT} -p ${pid}
+					${PROTECT} -p ${rc_pid}
 					;;
 				esac
 			fi