RTP: implement RTP socket abstraction layer
This abstraction layer is another step toward supporting a choice
between legacy Belledonne ortp and Osmocom-integrated twrtp.
Related: OS#6474
Depends: Ibda74c0dbfb163f5d0e3fb13f593a6e2c6817673 (libosmo-netif.git)
Change-Id: Ic945f6d753ec7f5c0ac5cecc9f71049464de7e8f
