* layer23: Avoid sending downlink AGCH/PCH fill frames over GSMTAP
* layer23: Identify AGCH from PCH on tx of GSMTAP downlink messages
* mobile: Send correct ARFCN in UL gsmtap
* trx_toolkit/data_msg.py: legacy flag is only for version 0x00
* trx_toolkit/data_msg.py: introduce header coding version 0x01
* trx_toolkit/data_msg.py: implement header version coding