Don't spam the user with error messages when non-root runs: rc-status

Without write permission cgroup_add_service can never work,
so check the pre-condition in the function for robustness.

There may need to be a logic fix as to why this is being run at
all during a status check, but the robustness fix is still
appropriate. If the logic is changed, we can ewarn before return.

Patch by Steve Long

https://bugs.gentoo.org/510636
--- a/rc-cgroup.sh	2013-12-08 05:35:00.000000000 +0000
+++ b/rc-cgroup.sh	2014-06-10 06:01:18.910038874 +0100
@@ -72,6 +72,7 @@
 
 cgroup_add_service()
 {
+	[ -w /sys/fs/cgroup ] || return
     # relocate starting process to the top of the cgroup
     # it prevents from unwanted inheriting of the user
     # cgroups. But may lead to a problems where that inheriting