attach: get personality through get_config command

attach: get personality through get_config command

Newer kernels optionally disallow reading /proc/$$/personality by
non-root users.  We can get the personality through the lxc command
interface, so do so.

Also try to be more consistent about personality being a signed long.
We had it as int, unsigned long, signed long throughout the code.

(This addresses bug
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1322067 :
3.15.0-1.x breaks lxc-attach for unprivileged containers)

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