cgroup updates: fix several bugs

1. add cgroup_get_subsys_abspath() which returns the absolute
path for a subsystem mount, and use that where needed to actually
set cgroup values

2. cgroup_devices_has_{allow,deny}: don't mix int and boolean
values.  Also, accept 'a *:* rwm" as any whitelist entry for
has_allow().

3. subsys_lists_match(): fix an off-by-one error in calculating
updated oldlen.  (we need to keep the extra char for '\0')

4. return -1, not 0, if lxc_cgroup_attach fails to open
/proc/self/cgroup.

Signed-off-by: Ubuntu <ubuntu@ip-10-181-158-15.ec2.internal>