gbproxy: Patch BSSGP P-TMSI in PAGING PS messages

Currently the P-TMSI IE in PAGING_PS is not patched.

This commit adds code to patch BSSGP P-TMSI IE in
gbproxy_patch_bssgp independently from the P-TMSI patching at the LLC
layer. It also extends gbproxy_update_link_state_dl to use the IMSI
to find the link_info if the TLLI is not present in the message.

Sponsored-by: On-Waves ehf
sgsn: Only send Detach Accept (MO) if power_off isn't set (TODO)

Currently, every time the SGSN received a Detach Request from the MS
via an established logical link, it is answered by a Detach Accept.
This violates the specification (GSM 04.08, 4.7.4.1.2 and .3), which
states, that it should only be sent, if "the detach type IE value
indicates that the detach request has not been sent due to switching
off".

This patch adds a conditional to limit the sending of Detach Accept
accordingly.

TODO:
  - add test

Sponsored-by: On-Waves ehf
sgsn: Handle Detach Requests even when there is no mmctx

Currently, when a Detach Request is received with an unknown TLLI,
it is answered by another Detach Request (!), even when a power_off
Type is used.

This patch uses gsm48_rx_gmm_det_req to handle the message instead.
So this function is changed to cope with a NULL mmctx. In that case
it doesn't unassign the llme, so this must be done manually
afterwards.

Sponsored-by: On-Waves ehf
sgsn: Avoid duplicated Attach Accept messages

Currently each received Ident Resp triggers an Attach Accept/Reject
if IMSI and IMEI are known. This has led to duplicated Attach Accept
messages when used with the gbproxy (IMSI acquisition active) and
with certain mobile equipment (iOS).

This patch modifies gsm48_rx_gmm_id_resp to discard Ident Resp messages
if all required information (IMEI and IMSI) has been gathered.

Ticket: OW#1322
Sponsored-by: On-Waves ehf
sgsn: Added TODOs (TODO)

TODO:
  - fix and remove TODOs
  - add test cases for Detach(power_off)
  - add test cases for Detach(reattach)
  - add test cases for PDP context deletion (OW#1311)
  - add test cases for Fix P-TMSI generation for repeated Attach Accept
    messages (OW#1322)
  - add test cases for Cancel pending timer in sgsn_mm_ctx_free
  - remove this commit

Sponsored-by: On-Waves ehf