Use libosmo-netif instead of socket interface.
Note: osmo_stream_srv_recv() is not used for now to keep as much of existing
code as possible and make troubleshooting easier.
Related: OS#5568
Change-Id: Idc2e60af1010783e555e61b114ae61f55a89d890
As part of preparation for libosmo-netif migration let's move common SMPP code
into separate build-time library and use it for both smpp_mirror and OsmoMSC
renaming the files if necessary.
While at it we also fix id/password legth limits in smpp_mirror and drop unused
fields from ESME struct.
Related: OS#5568
Change-Id: I61910651bc7c188dc2fb67d96189a66a47e7e8fb