Add osmo-euse-demo as minimalistic test of a External USSD (EUSE) handler

This is a small program which simply echo's the USSD request message it
gets in a quote back to the sender.  Its purpose is to illustrate how
EUSEs can be implemented using libosmo-gsup-client.

Change-Id: I3fb8554ca329cb609c591058254117006f665e73
2 files changed