add option to send SCCP CR without payload

It is reported that a third-party SGSN is rejecting SCCP CR when the
SCCP message part exceeds a certain length. The solution is to first
send an SCCP CR without payload, and send the payload in a DT later.

Add config option

  hnbgw
   sccp cr max-payload-len <0-999999>

If the RANAP payload surpasses the given length, osmo-hnbgw will first
send an SCCP CR without payload, cache the RANAP payload, and put that
in an SCCP DT once the SCCP CC is received.

The original idea was to limit the size of the entire SCCP part of the
message, but I'm currently not sure how to determine that without
copying much of the osmo_sccp code. I figured using a limit on the RANAP
payload is sufficient. To avoid the error with above third-party SGSN,
the easy solution is to set max-payload-len to 0, so that we always get
a separate SCCP CR without payload.

Related: SYS#5968
Related: I827e081eaacfb8e76684ed1560603e6c8f896c38 (osmo-ttcn3-hacks)
Change-Id: If0c5c0a76e5230bf22871f527dcb2dbdf34d7328
---
M include/osmocom/hnbgw/context_map.h
M include/osmocom/hnbgw/hnbgw.h
M include/osmocom/hnbgw/hnbgw_rua.h
M src/osmo-hnbgw/context_map.c
M src/osmo-hnbgw/hnbgw.c
M src/osmo-hnbgw/hnbgw_cn.c
M src/osmo-hnbgw/hnbgw_rua.c
M src/osmo-hnbgw/hnbgw_vty.c
8 files changed, 123 insertions(+), 14 deletions(-)