apparmor: deny writes to most of /proc/sys (v2)

Allow writes to kernel.shm*, net.*, kernel/domainname and
kernel/hostname,

Also fix a bug in the lxc-generate-aa-rules.py script in a
path which wasn't being exercised before, which returned a
path element rather than its child.

Changelog (v2): remove trailing / from block path

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>