Changes

mtp: Support MTP-TRANSFER.req/ind of raw IPA messages

This is needed ie. in SCCPLite, where MGCP and CTRL messages need to be
senti/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.

Related: SYS#6880
Change-Id: I9fedb26ccd3434fc7f272feb3c45cf4bdb80c7ae