attach: join apparmor profile

This patch enables lxc-attach to join the profile of the container it
is attaching to.  Builds/runs fine with apparmor enabled and disabled.

Export new aa_get_profile(), and use it for attach_apparmor, but also
handle profile names longer than 100 chars in lxc_start apparmor
support.

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