sgsn: Be more tolerant with state and SUSPEND/RESUME

Currently, when a BSSGP SUSPEND is received and the corresponding MM
context is already in the state GMM_REGISTERED_SUSPENDED, a
SUSPEND_NACK is returned which is not covered by GSM 08.18, 7.4.1.
The same goes for RESUME in the state GMM_REGISTERED_NORMAL.

This commit changes gprs_gmm_rx_suspend and gprs_gmm_rx_resume to not
complain (and thus answer a NACK) when the MM context is either in
GMM_REGISTERED_SUSPENDED or GMM_REGISTERED_NORMAL.

Note that GSM 08.18, 7.4.1 and 7.5.1 only mention to send an NACK if
the MS is not known. Even with this patch, the SGSN returns a NACK if
either the MS is unknown _or_ the MM context ist not in one of the
state GMM_REGISTERED_SUSPENDED and GMM_REGISTERED_NORMAL.

Sponsored-by: On-Waves ehf
sgsn: Add logging to connect_timer_cb (TODO)

TODO:
  - LOGLevel ?

Sponsored-by: On-Waves ehf
sgsn: Add PDP info to subscriber data (TODO)

TODO:
  - commit message
  - raw binary representation ???
  - tests

Sponsored-by: On-Waves ehf
sgsn: Do not die in _bssgp_tx_dl_ud on TLLI mismatch (TODO)

Currently an OSMO_ASSERT fails, if the message's and the mmctx's
TLLIs do not match.

This commit turns the assertion into an ERROR log message and uses
the default values for IMSI, DRX, and RA CAP instead of the MM
context values in this case.

TODO:
  - find the real source for the problem
  - add test cases derived from what is being described in the ticket

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

TODO:
  - fix and remove TODOs
  - add test cases for Detach(reattach)
  - add test cases for PDP context deletion (OW#1311)
  - add test cases for Cancel pending timer in sgsn_mm_ctx_free
  - remove this commit

Sponsored-by: On-Waves ehf