Add attach.[ch]: Helper functions for lxc-attach

Add attach.[ch]: Helper functions for lxc-attach

The following helper functions for lxc-attach are added to a new file
attach.c:
 - lxc_proc_get_context_info: Get cgroup memberships, personality and
   capability bounding set from /proc for a given process.
 - lxc_proc_free_context_info: Free the data structure responsible
 - lxc_attach_proc_to_cgroups: Add the process specified by the pid
   parameter to the cgroups given by the ctx parameter.
 - lxc_attach_drop_privs: Drop capabilities to the capability mask given in
   the ctx parameter.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>