TCH DL, FR & EFR: reshape SIDs per 06.31/06.81 section 5.1.2

Section 5.1.2 of GSM 06.31 & 06.81 specifies the expected shape of
radio downlink for FR & EFR in the presence of SIDs: one SID frame
after each talkspurt (after speech frames), and one SID frame in
every SACCH-aligned position every 480 ms, or if the actual SACCH-
aligned position is taken up by FACCH, then just one SID frame as
soon as possible after that FACCH - and no transmitted SID frames
in other positions.

This prescribed shape of call leg B DL won't happen on its own if
the RTP stream comes from call leg A UL in a TrFO scenario - whichever
SID frames are present will be in wrong positions for leg B DL,
and even in the case of transcoded calls the responsibility for DL
SID shaping cannot be placed on the RTP stream source because that
source won't know where SACCH alignment will lie.  Therefore,
the necessary DL SID reshaping has to be done in the RTP stream
receiver in OsmoBTS.

Related: OS#5996
Change-Id: I924ab21952dcf8bb03ba7ccef790474bf66fc9e5
---
M include/osmo-bts/lchan.h
M src/common/l1sap.c
2 files changed, 142 insertions(+), 0 deletions(-)