Support abstract namespaces with UNIX sockets

[herwinw]

Another one for the long list of things I want to do: abstract namespaces for UNIX sockets.

A selective copy-paste from https://www.man7.org/linux/man-pages/man7/unix.7.html:

an abstract socket address is distinguished (from a
pathname socket) by the fact that sun_path[0] is a null
byte ('\0'). The socket's address in this namespace is
given by the additional bytes in sun_path that are covered
by the specified length of the address structure. (Null
bytes in the name have no special significance.) The name
has no connection with filesystem pathnames. When the
address of an abstract socket is returned, the returned
addrlen is greater than sizeof(sa_family_t) (i.e., greater
than 2), and the name of the socket is contained in the
first (addrlen - sizeof(sa_family_t)) bytes of sun_path.

A simple example in Ruby:

require 'socket'

path = "\x00abstract"
server = UNIXServer.new(path)
client = UNIXSocket.new(path)
client.write("Hello")
socket = server.accept
puts socket.recvfrom(5).first
socket.close
client.close
server.close

I completely skipped this for now to get the specs passing, but we should support this too. And preferably write some specs for them as well.