Check for protocol extension bit in message type at a central location
Bits 8 and 7 of message type can be used as sequence number or have specific default values. The added check will verify the bits according to the prococol descriptor and Release version.
The Rules for these bits are described in GSM TS 124.007, clause 11.2.3.2.1 for Release <= 98 and 11.2.3.2.2 for Release >= 99.
There is also no need to clear bit 7 in message type for these protocols, as it is done already in gsm48_hdr_msg_type().
Some unit tests fail without fixes that are included in this patch.
