change nih_fatal to nih_warn in most cases

Also don't return error on mkdir of an existing cgroup.
(But don't chown to the caller)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
change asserts to simple checks

we don't want the daemon to exit out due to bad client output,
just to not go on and choke on it.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>