bts: Remove unused fields from the trx structure

These have either never been used or were replaced with a
more generic approach.
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
sgsn: Add support for authentication triplets (TODO)

TODO:
  - commit message
  - tests

Sponsored-by: On-Waves ehf
sgsn: Work on 2 phase authorization (WIP)

This implements the MAP way of subscriber validation when the MS
tries an Attach Request:
  1. perform authentication (optionally invoke the sendAuthInfo
  procedure), starts the AuthAndCiph procedure
  2. perform update location
  3. insert subscriber data
  4. finish the update location
  5. Attach Accept / Attach Reject

This is currently accessible via the VTY interface.

TODO:
  - Apply this to (periodic) RA Update Req
  - Delete Subscriber Data
  - Improve on procedure collisions (repeated Attach Req is sometimes
    not handled properly)
  - PDP info
  - Commit cleanup

Sponsored-by: On-Waves ehf
gprs: Move TLV parser functions to gprs_utils.c and rename them

Currently the parser functions for single information elements are
defined within gprs_gb_parse.c and not exported explicitely. In
addition they are named like libosmocore's TLV parser functions and
do not have a proper name prefix. Since it is planned to use them for
other protocols, they need to be globally accessible.

This patch moves them to gprs_utils.c and renames them.

The new names are:
    lv_shift             -> gprs_shift_lv
    v_fixed_shift        -> gprs_shift_v_fixed
    lv_shift             -> gprs_shift_lv
    v_fixed_shift        -> gprs_shift_v_fixed

In the long term, these functions should be moved to libosmocore (and
renamed again).

Sponsored-by: On-Waves ehf
gprs: Add gprs_shift_tlv function

This function is similar to gprs_match_tlv with the exception, that
the tag is not compared but returned in *tag instead.

Sponsored-by: On-Waves ehf
sgsn/doc: Add protocol specification for remote subscriber update

This is the first version of the specification for the GPRS
Subscriber Update Protocol.

Sponsored-by: On-Waves ehf
gprs: Add encoder/decoder for the Subscriber Update Protocol (WIP)

Status: compiles and test passes

TODO:
  - Add commit message

Ticket: OW#????
Sponsored-by: On-Waves ehf