attach: mount a sane prox for LSM setup

attach: mount a sane prox for LSM setup

To set lsm labels, a namespace-local proc mount is needed.

If a container does not have a lxc.mount.auto = proc set, then
tasks in the container do not have a correct /proc mount until
init feels like doing the mount.  At startup we handlie this
by mounting a temporary /proc if needed.  We weren't doing this
at attach, though, so that

lxc-start -n $container
lxc-wait -t 5 -s RUNNING -n $container
lxc-attach -n $container -- uname -a

could in a racy way fail with something like

lxc-attach: lsm/apparmor.c: apparmor_process_label_set: 183 No such file or directory - failed to change apparmor profile to lxc-container-default

Thanks to Chris Townsend for finding this bug at
https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1452451

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