sgsn: Send detach(re-attach) instead of gmm status if TLLI unknown (TODO)

The osmo-sgsn sends Status messages (or nothing in case of non
GMM/GSM) when the TLLI is unknown. This prevents the MS from
reconnecting.

This experimental patch initiates a MT detach procedure to force a
re-attach to set up a valid LLE context.

TODO:
  - test cases

Note that the MS can be put into REGISTERED state after initially
wanting to detach itself, since it will receive a Detach Req
(re-attach) after the SGSN or gbproxy (P-TMSI patching enabled) has
been restarted. This same behaviour has been observed with another
SGSN.

Sponsored-by: On-Waves ehf
sgsn: Added TODOs (TODO)

TODO:
  - fix and remove TODOs
  - remove this commit

Sponsored-by: On-Waves ehf