sccplite: Handle MGCP/CTRL over SCCPLite multiplex using MTP-TRANSFER.req/ind

This is needed ie. in SCCPLite, where MGCP and CTRL messages need to be
sent/received over the same IPA multiplex TCP conn as the SCCPLite
(SCCP/IPA) one towards a given MSC.

Prior to this, osmo-bsc-sccplite was doing lots of manual
libosmo-sigtran APIs to obtain a route, an AS, and ASP, sending over an
ASP, etc. Similary, the rx_unknown callback was added to libosmo-sigtran
to hook into the ASP level. Tons of stuff which should really be done
properly inside libosmo-sigtran and provided over the existing public
SAPs, as this patch proposes.

With the proposed approach, an SCCPLite client (eg. osmo-bsc), besides
setting up an sccp_user to handle SCCP, also sets an ss7_user (MTP SAP)
with the newly introduced new Service Indicator (SI) MTP_SI_NI11_OSMO_IPA
to be able to manage the IPA[MGCP/CTRL] traffic towards a given MTP peer
(SCCPLite client, ie. MSC).

Related: SYS#6880
Depends: libosmo-sigtran.git Change-Id I9fedb26ccd3434fc7f272feb3c45cf4bdb80c7ae
Change-Id: I567ed206eab1add21836bfd937f3790d3d7a00d7
---
M TODO-RELEASE
M include/osmocom/bsc/bsc_msc_data.h
M include/osmocom/bsc/osmo_bsc_sigtran.h
M src/osmo-bsc/bsc_ctrl.c
M src/osmo-bsc/osmo_bsc_mgcp.c
M src/osmo-bsc/osmo_bsc_sigtran.c
6 files changed, 157 insertions(+), 116 deletions(-)