Enforce consistent limits of daemons run from rc.subr:

* Allow the user to configure the login class to use in rc.conf
  by using {daemon}_login_class, which;
* Use the daemon class by default;
* .. and then use 'limits' to set the login class so it works both
  via init at startup (which runs this in 'daemon' class) and via
  whichever root environment (eg command line, other daemons, etc.)

Reviewed by:	dteske
Differential Revision:	https://reviews.freebsd.org/D3630

etc/rc.subr

@@ -768,6 +768,8 @@ check_startmsgs()
 #
 #	${name}_prepend	n	Command added before ${command}.
 #
+#	${name}_login_class n	Login class to use, else "daemon".
+#
 #	${rc_arg}_cmd	n	If set, use this as the method when invoked;
 #				Otherwise, use default command (see below)
 #
@@ -942,7 +944,7 @@ run_rc_command()
 	    _nice=\$${name}_nice	_user=\$${name}_user \
 	    _group=\$${name}_group	_groups=\$${name}_groups \
 	    _fib=\$${name}_fib		_env=\$${name}_env \
-	    _prepend=\$${name}_prepend
+	    _prepend=\$${name}_prepend	_login_class=\${${name}_login_class:-daemon}
 
 	if [ -n "$_user" ]; then	# unset $_user if running as that user
 		if [ "$_user" = "$(eval $IDCMD)" ]; then
@@ -1050,6 +1052,9 @@ $command $rc_flags $command_args"
 				fi
 			fi
 
+					# Prepend default limits
+			_doit="limits -C $_login_class $_doit"
+
 					# run the full command
 					#
 			if ! _run_rc_doit "$_doit"; then