CSD NT modes: transmit properly aligned RLP frames on DL

There are two levels of alignment inside clearmode RTP packets
carrying CSData per TS 48.103 section 5.6:

1) Alignment of 2 or 4 V.110 (T) or pseudo-V.110 (NT) frames within
   one RTP packet of 160 octets of an imaginary ISDN B channel;

2) For NT modes only, alignment of 4 pseudo-V.110 frames to form
   a single 240-bit RLP frame.

Per previous patch, alignment 1 is to be treated as mandatory for
RTP transport inside an Osmocom network.  Alignment 2 _could_ be
made mandatory for TCH/F9.6 NT, but the same is not possible for
TCH/[FH]4.8 NT: in the best case of half-alignment, alternating
RTP packets will carry alternating halves of RLP frames.

Implemented solution: allow arbitrary state of alignment 2
(aligned or misaligned) in the incoming RTP stream for all CSD NT
modes, and perform the necessary alignment internally.

This approach is consistent with the world of E1 BTS: a TRAU in
data mode is responsible for alignment 1 (with 20 ms TRAU frames
taking the place of our clearmode RTP packets), but only the BTS can
perform alignment 2, as the TRAU is agnostic to T vs NT distinction.

Related: OS#6579
Change-Id: Idaebfce6da13b23ba265a197502712d83991873e
---
M include/osmo-bts/csd_rlp.h
M include/osmo-bts/lchan.h
M src/common/csd_rlp.c
M src/common/l1sap.c
4 files changed, 301 insertions(+), 29 deletions(-)