sgsn: Reorganize and fix gsm48_gmm_authorize (TODO)

Currently the order of the 'if' clauses in gsm48_gmm_authorize
doesn't match the order in which the conditional parts are entered.
This makes it difficult to maintain. In addition the t3350_mode is
not stored in every path, so that this information is lost when the
identification procedure is started. Since the default value
coincidentally is GMM_T3350_MODE_ATT, this doesn't hurt for Attach
Requests which are the only messages that initially trigger the
authentication yet.

This patch changes the order of the 'if' clause to match the
processing order, it removes the t3350_mode parameter entirely and
introduces a mm->pending_req field. The latter must be set when the
request that causes the authorization before calling
gsm48_gmm_authorize. The gprs_t3350_mode enum is extended by
GMM_T3350_MODE_NONE (value 0, which is the default) to make it
possible to detect related initialisation errors or race conditions.

TODO:
 - There isn't much difference between t3350_mode (refers to the
   timer) and pending_req (refers to the procedure),
   where mm->T == 3350 => mm->t3350_mode == f(mm->pending_req).
   Check whether one of them can be dropped.

Sponsored-by: On-Waves ehf
sgsn/test: Move message sending to send_0408_message

This replaces serveral occurences of duplicated code for message
creation and sending (passing to gsm0408_gprs_rcvmsg) into a single
function. In addition, the sgsn_tx_counter is always reset within
send_0408_message to simplify the code that checks for the number of
messages sent.

Sponsored-by: On-Waves ehf
sgsn/test: Add test_gmm_attach

This test checks the attach procedure until the Attach Complete is
received.

Note that authorization and GMM state updates are not working
properly yet.

Sponsored-by: On-Waves ehf
sgsn: Cleanup GMM state transitions

Currently the GMM state is set to GMM-REGISTERED when an Attach
Accept or a RA Update Accept message is sent, even if a new P-TMSI is
included. In this case 04.08 requires (see 4.7.3.1.3 and 4.7.5.1.3),
that the state is set to GMM-COMMON-PROCEDURE-INITIATED when the
Accept is sent. When the Complete is received, the SGSN shall set
the state to GMM-REGISTERED.

This patch modifies the state updates accordingly.

Sponsored-by: On-Waves ehf
sgsn: Move IMSI authorization to gsm48_gmm_authorize

Currently the IMSI is only checked immediately when an Attach Request
is received that contains an IMSI IE. If it contains a P-TMSI
instead, access is always granted.

This commit moves the IMSI check to gsm48_gmm_authorize where it is
applied when IMSI and IMEI have been acquired. This fixes the
authorization when the Attach Accept doesn't contain an IMSI.

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 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
sgsn: Make authorization asynchronous (EXPERIMENTAL)

Currently the authorization of an IMSI is done by checking ACLs
synchronously which is not feasible when the subscriber data has to
be retrieved from an external source.

This patch changes this by using a callback when the information is
available. This is also done when only ACL are checked, in this case
the callback is invoked from within sgsn_auth_request(). The callback
function sgsn_update_subscriber_data calls either
gsm0408_gprs_access_granted or gsm0408_gprs_access_denied.

The change has been successfully tested with a single phone (E71).

Sponsored-by: On-Waves ehf
sgsn: Remove unused static functions from gprs_gmm.c

The following functions are not being used:
 - gsm48_tx_gmm_status_oldmsg
 - gsm48_tx_sm_status_oldmsg

This patch removes the function definitions.

Sponsored-by: On-Waves ehf