Changes

fix MGCP compat with osmo-mgw <= 1.12.2: CRCX in recvonly

Fix a recently introduced problem with MGCP to osmo-mgw:
Send the first CRCX in recvonly mode, not sendrecv. osmo-hnbgw always
sends an additional MDCX including sendrecv mode anyway.

osmo-mgw currently forbids sending an initial CRCX in connection mode
'sendrecv', with this error message:

  DLMGCP ERROR endpoint:rtpbridge/2@mgw CI:7F4C8EDD CRCX: selected connection mode type requires an opposite end! (mgcp_protocol.c:1090)

I am submitting an osmo-mgw patch to not fail there, but we want to and
can easily be compatible with current and earlier osmo-mgw:

Sending the initial CRCX in sendrecv was introduced in commit:

	"drop legacy hack: do not start MGW endp in loopback mode"
	da7d33e2841a2be94fd3364dc44abf8068669998
	I0eca75d7abf66f8b9fde9c68ec10d4265f64a189
	This patch has not been part of a release yet.

The intention of that commit was to get away from loopback mode. The
logical mode to pick instead indeed is sendrecv, but by that osmo-hnbgw
triggers above osmo-mgw error.

Related: SYS#6974 SYS#6907
Related: osmo-mgw Ic089485543c5c97a35c7ae24fe0f622bf57d1976
Change-Id: I004f96ae36774ceb33f177c9f58f820fefa3ca14