rc: add ${name}_setup script support

Run a service-based setup script before running the start command.
Useful for automatic configuration file generation.

Reviewed by:	https://reviews.freebsd.org/D36006

(cherry picked from commit c9be47b34dd847da1d4ab0feee4a6100b2c5ea0d)

libexec/rc/rc.subr

@@ -782,6 +782,8 @@ check_startmsgs()
 #
 #	${name}_prepend	n	Command added before ${command}.
 #
+#	${name}_setup	n	Command executed before ${command}.
+#
 #	${name}_login_class n	Login class to use, else "daemon".
 #
 #	${name}_limits	n	limits(1) to apply to ${command}.
@@ -966,8 +968,8 @@ run_rc_command()
 	    _group=\$${name}_group	_groups=\$${name}_groups \
 	    _fib=\$${name}_fib		_env=\$${name}_env \
 	    _prepend=\$${name}_prepend	_login_class=\${${name}_login_class:-daemon} \
-	    _limits=\$${name}_limits    _oomprotect=\$${name}_oomprotect \
-	    _env_file=\$${name}_env_file
+	    _limits=\$${name}_limits	_oomprotect=\$${name}_oomprotect \
+	    _setup=\$${name}_setup	_env_file=\$${name}_env_file
 
 	if [ -n "$_env_file" ] && [ -r "${_env_file}" ]; then	# load env from file
 		set -a
@@ -1122,6 +1124,12 @@ $command $rc_flags $command_args"
 				fi
 			fi
 
+			if [ -n "$_setup" ]; then
+				if ! _run_rc_doit "$_setup"; then
+					warn "failed to setup ${name}"
+				fi
+			fi
+
 					# Prepend default limits
 			_doit="$_cd limits -C $_login_class $_limits $_doit"
 
@@ -2138,7 +2146,7 @@ check_kern_features()
 # check_namevarlist var
 #	Return "0" if ${name}_var is reserved in rc.subr.
 
-_rc_namevarlist="program chroot chdir env flags fib nice user group groups prepend"
+_rc_namevarlist="program chroot chdir env flags fib nice user group groups prepend setup"
 check_namevarlist()
 {
 	local _v