CC: don't start guard timer on mid-call MNCC messages

The intent of the guard timer is to clear hung or stuck states
during call setup or teardown.  However, there are some MNCC
messages that will be exchanged between OsmoMSC (passing CC
messages to and from the MS) and the external MNCC agent during
the active call state, not related to setup or teardown: DTMF
start and stop, plus call hold and retrieve operations for call
waiting.  Unpatched OsmoMSC restarts the guard timer on every
received MNCC message, even those that pass through to CC without
affecting any state, and the result is breakage for users.

Consider the case of an IVR where you have to press some DTMF keys
before you can be transferred to a human operator.  You press the
needed keys, get the human operator, and start talking.  Then
3 minutes into your conversion (default guard timer duration)
your call unceremoniously disconnects without any warning.

Fix: look at the MNCC message type, and skip the call to start
the guard timer for known-benign MNCC messages.

Change-Id: Ibe2dd53f8e9e06d175b64df67d2a2e3e2d4155aa
---
M src/libmsc/gsm_04_08_cc.c
1 file changed, 49 insertions(+), 1 deletion(-)