lxc-attach: Drop privileges when attaching to container unless requested otherwise

lxc-attach: Drop privileges when attaching to container unless requested otherwise

lxc-attach will now put the process that is attached to the container into
the correct cgroups corresponding to the container, set the correct
personality and drop the privileges.

The information is extracted from entries in /proc of the init process of
the container. Note that this relies on the (reasonable) assumption that the
init process does not in fact drop additional capabilities from its bounding
set.

Additionally, 2 command line options are added to lxc-attach: One to prevent
the capabilities from being dropped and the process from being put into the
cgroup (-e, --elevated-privileges) and a second one to explicitly state the
architecture which the process will see, (-a, --arch) which defaults to the
container's current architecture.

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