Finish the work started by Daniel in 2012, by using abstract local
AF_UNIX sockets on platforms that support them (Linux, Android).
Fix all sorts of bugs and issues that prevented the existing MDP and
Monitor client and server code from working with abstract socket names.
Handle selection between normal & abstract sockets in a single place.
overlay_mdp.c tried to have both but this seems like a bug so I've removed the second socket (needs tests).
This consistently handles abstract vs normal sockets with a single switch.
overlay mdp tries to have both (abstract & normal) but the code paths seem different, I have consolidated them.
Needs more testing.