sccplite: Use mgwpool config to set up socket forwarding IPA-MGCP from MSC to MGW

In SCCPlite, the BSC receives the CN-side MGCP from the MSC through an
IPA conn, and it then forwards those messages to its co-located MGW
through a rawUDP socket created at startup.

This forwarding UDP socket still relied exclusively on the "mgw.conf"
struct which was filled only by the old VTY interface which was been
deprecated lately.

This patch moves the mgw-pool setup before the msc setup so that if the
VTY config file still uses the old VTY, the single MGW is added to the
MGW pool through mgcp_client_pool_register_single(). It then simply
picks the first available MGW from the pool when creating the raw UDP
MGCP-forwarding socket.

This means SCCPLite is still left with supporting only 1 MGW. If more
than one MGW is defined in the pool, then when the call is being set up
a different MGW could be picked from the pool while the CN-side MGCP
would still be sent to the MGW pool selected at osm-bsc startup.

This limitation coul be removed later on  by adding a new VTY command
under the "msc" to pin calls for an MSC to an MGW with a given "mgw_nr"
from the pool, and that same MGW be looked for in the pool every time a
new call is being established.
Another possibility would be to avoid creating the "connected" UDP
socket at osmo-bsc startup, and rather use it in non-connected mode and
transmit (sendto) using the mgcp_client remote address during call
establishment.
In any case, this is left as future excercise since so far there hasn't
been any need for multiple MGWs in SCCPLite setups.

Related: SYS#5987
Change-Id: If105dee52b8d36161c759f979eaef4579b47d365
---
M src/osmo-bsc/osmo_bsc_main.c
M src/osmo-bsc/osmo_bsc_msc.c
2 files changed, 16 insertions(+), 10 deletions(-)