Add kernel uid/gid contexts

Add kernel uid/gid contexts

Add the following kernel tracer context fields:
  - uid
    Real user ID

  - euid
    Effective user ID

  - uid
    Real user ID

  - suid
    Saved set-user ID

  - gid
    Real group ID

  - egid
    Effective group ID

  - sgid
    Effective saved set-user group ID

  - 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>