MEDIUM: unix: implement support for Linux abstract namespace sockets

MEDIUM: unix: implement support for Linux abstract namespace sockets

These sockets are the same as Unix sockets except that there's no need
for any filesystem access. The address may be whatever string both sides
agree upon. This can be really convenient for inter-process communications
as well as for chaining backends to frontends.

These addresses are forced by prepending their address with "abns@" for
"abstract namespace".