codec: add SID preening functions for FR & EFR

Those network elements which receive a stream of codec frames that
may come from the uplink of GSM call A and which are responsible
for preparing the frame stream for the downlink of GSM call B
(OsmoMGW feeding TRAU-DL, or OsmoBTS receiving RTP and feeding DL
to its PHY) must be prepared for the possibility that their
incoming frame stream may contain corrupted SID frames, presumably
from bit errors on radio link A.  Per the rules of section 6.1.1
of GSM 06.31 for FR and GSM 06.81 for EFR, SID frames with just one
errored bit are still to be accepted as valid, whereas frames with
more corrupted bits which are still recognizable as SID are classified
as invalid SID.

In the case of a TrFO call, the entity switching from leg A UL to
leg B DL is responsible for *not* transmitting invalid SID frames
on the destination leg (they should be treated like BFIs), and any
deemed-valid SID frames that are forwarded should be preened,
correcting that one bit error they may exhibit.  The functions
added here provide that functionality.

Change-Id: Iec5c1f2619a82499f61cb3e5a7cd03ff0f020ad8
---
M include/osmocom/codec/codec.h
M src/codec/gsm610.c
M src/codec/gsm660.c
3 files changed, 110 insertions(+), 0 deletions(-)