If the rc.conf(5) variable for a script is not enabled do not fail

silently. Display a message that the command wasn't run and make
possible suggestions for what to do.

PR:	   conf/118770
MFC after: 1 week

etc/rc.subr

@@ -606,6 +606,9 @@ run_rc_command()
 					#
 	if [ -n "${rcvar}" -a "$rc_arg" != "rcvar" ]; then
 		if ! checkyesno ${rcvar}; then
+			echo -n "Cannot '${rc_arg}' $name. Set ${rcvar} to "
+			echo -n "YES in /etc/rc.conf or use 'one${rc_arg}' "
+			echo "instead of '${rc_arg}'."
 			return 0
 		fi
 	fi