Add uid/gid contexts

Add uid/gid contexts

Add a context for each available kernel user and group IDs

  * uid : real user ID
  * euid : effective user ID
  * suid : saved set-user ID

These are the IDs as seen in the initial user namespace, see
credentials(7) for details on each type.

Also add a "virtual" version of each type with the "v" prefix that
are the IDs as seen in the current user namespace.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>