The recently added cmp_sockaddr() function does not call stat(2) any
more to compare local AF_UNIX socket address paths, so not it is stable
enough to use for ordering sockaddr structs.
New function: real_sockaddr() converts the file path of a local AF_UNIX
file socket using realpath(3). The MDP client uses it on the sender
address of every MDP reply packet it receives to ensure that symlinks in
the instance path do not cause MDP client failures.
Rename recently added socket_setname() function: make_local_sockaddr().
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).
