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

libexec/rc/rc.subr

@@ -52,6 +52,14 @@ ID="/usr/bin/id"
 IDCMD="if [ -x $ID ]; then $ID -un; fi"
 PS="/bin/ps -ww"
 JID=0
+# rc_service provides the path to the service script that we are executing.
+# This is not being set here in an execution context, necessarily, so it's
+# really just a reasonable guess, and it will get overwritten later if
+# we are executing from some other means than direct execution by service(8)
+# or manual invocation of the service script.  The prime example of this is
+# during system startup, all rc scripts will be invoked via /etc/rc, so
+# run_rc_script will overwrite rc_service with the file being sourced.
+rc_service="$0"
 
 #
 #	functions
@@ -879,6 +887,9 @@ check_startmsgs()
 #			by $flags from the environment.
 #			This variable may be changed by the precmd method.
 #
+#       rc_service      Path to the service being executed, in case it needs to
+#                       re-invoked.
+#
 #	rc_pid		PID of command (if appropriate)
 #
 #	rc_fast		Not empty if "fast" was provided (q.v.)
@@ -1382,6 +1393,7 @@ run_rc_script()
 		required_vars
 	eval unset ${_arg}_cmd ${_arg}_precmd ${_arg}_postcmd
 
+	rc_service="$_file"
 	case "$_file" in
 	/etc/rc.d/*.sh)			# no longer allowed in the base
 		warn "Ignoring old-style startup script $_file"

share/man/man8/rc.subr.8

@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 7, 2018
+.Dd January 5, 2019
 .Dt RC.SUBR 8
 .Os
 .Sh NAME
@@ -771,7 +771,7 @@ The following variables are available to the methods
 as well as after
 .Ic run_rc_command
 has completed:
-.Bl -tag -width ".Va rc_flags" -offset indent
+.Bl -tag -width ".Va rc_service" -offset indent
 .It Va rc_arg
 Argument provided to
 .Ic run_rc_command ,
@@ -785,6 +785,8 @@ unless overridden by the environment variable
 This variable may be changed by the
 .Ar argument Ns Va _precmd
 method.
+.It Va rc_service
+Path to the service script being executed, in case it needs to re-invoke itself.
 .It Va rc_pid
 PID of
 .Va command