osmo_trx: add TRXDv2 PDU dissection support
This Pull Request brings support for version 2 of the TRXD protocol, which we use in Osmocom for encapsulating Um-interface bursts and sending them over UDP/IP. Protocol documentation can be found here: http://ftp.osmocom.org/docs/latest/osmotrx-usermanual.pdf (chapter 19).