Cellular Network Infrastructure

osmo-msc.git and osmo-sgsn.git both use GSUP.
The GSUP protocol definition is in libosmocore already, but both osmo-sgsn and osmo-msc duplicate the gsup_client code.
The gsup_client also depends on oap_client. OAP is currently not used by us but we should keep it active for future use.

gsup_client uses libosmo-abis/ipa as IPA Multiplex implementation.
Hence gsup_client as well as oap_client cannot move to libosmocore, but need to move "at least" to libosmo-abis.
(libosmo-abis/ipa uses libosmocore/gsm/ipa, but the server link code is in libosmo-abis.)

Semantically, libosmo-abis is not the right place for GSUP and OAP: they are not at all related to the Abis interface.

Clarify: what should move where? Then move it to drop the gsup_client code dup between osmo-msc and osmo-sgsn.