Add UST uid/gid contexts

Add the following userspace tracer context fields:
  - vuid
    Virtual real user ID: real user ID as seen from the point of view of
    the current user namespace

  - veuid
    Virtual effective user ID: effective user ID as seen from the point of
    view of the current user namespace

  - vsuid
    Virtual saved set-user ID: saved set-user ID as seen from the point of
    view of the current user namespace

  - vgid
    Virtual real group ID: real group ID as seen from the point of view of
    the current user namespace

  - vegid
    Virtual effective group ID: effective group ID as seen from the point of
    view of the current user namespace

  - vsgid
    Virtual saved set-group ID: saved set-group ID as seen from the point of
    view of the current user namespace

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>