rc.subr: Provide rc_service variable for service scripts

Some rc scripts in ports (e.g. uwsgi, apache, openvpn) allow for
'application profiles' that usually require the rc script to be invoked
again for each active profile. Because there's no consistent way to
determine the path because it differs between manual/service(8) invocations
and /etc/rc invocations, this leads to patterns like these:

• www/uwsgi hardcodes the script path
• security/openvpn guesses either $_file or $0 based on $0 = /etc/rc

Instead of forcing rc scripts to guess, provide an rc_service variable to
the scripts that gets set appropriately both for direct execution or when a
script is being executed via run_rc_script (e.g. /etc/rc).

This is our analog of an OpenRC variable with the same name, different case
(RC_SERVICE).

PR: 234614
Reported by: koobs
Reviewed by: dteske, jilles
MFC after: 3 days