Changes

CSD RTP: verify alignment of V.110 frames

Since the beginning of CSD implementation, OsmoBTS has operated
with an implicit (unstated) policy that V.110 frames must be
perfectly aligned within received clearmode RTP packets - a requirement
which is NOT set anywhere in TS 48.103 or any of the other specs
it references.  This design policy is sensible from the standpoint
of implementation complexity (both OsmoBTS and osmo_trau2rtp emit
such perfectly aligned packets; if someone is building a gateway
between an Osmocom GSM network and ISDN-style external networks,
that gateway can act as the aligner), but it should be explicit
rather than implicit.

Check V.110 frame alignment in received clearmode RTP packets,
and reject packets that fail this alignment check.

Change-Id: Icd704dc7fa02e60074efc8a29ad7e42ebdf63783