gbproxy: Add gprs_gb_message_name function

This function tries to get an accurate name for the message even if
the parsing has been aborted due to message errors.

The patch also moves the settings of the BSSGP related fields in
parse_ctx from behind to the front of bssgp_tlv_parse, to get more
information in the case of failure. This is now consistent with the
handling of the llc and g48_hdr fields.

Id addition, gprs_gb_log_parse_context now uses the new function to
derive a more accurate message name.

Ticket: OW#1307
Sponsored-by: On-Waves ehf
gbproxy: Pass the log level as argument to gprs_gb_log_parse_context

Currently, the log level is always LOGL_DEBUG. In case of errors it
would be helpful to use a higher log level.

This patch adds a log_level parameter to gprs_gb_log_parse_context to
let the caller decide about the level.

Ticket: OW#1307
Sponsored-by: On-Waves ehf
gbproxy: Log more information on parse errors

To get a clue which message cause the error without having to enable
LOGL_DEBUG, information about how far the parser came (message name,
parsed fields) is logged with LOGL_NOTICE along with a full hexdump
of the message.

Ticket: OW#1307
Sponsored-by: On-Waves ehf
gbproxy/test: Add expect_msg checks to test_gbproxy_keep_info (TODO)

This patch adds assertions to check the LLC/GMM message received from
the gbproxy by the test framework within the function
test_gbproxy_keep_info.

It also fixes the source address of the DETACH ACC messages.

TODO:
 - more tests

Sponsored-by: On-Waves ehf
gbproxy: Patch BSSGP P-TMSI in PAGING PS messages (TODO)

TODO:
  - commit message
  - finish test case

Sponsored-by: On-Waves ehf
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