support datagram unix domain sockets

test/unixudpsrvr.lua

@@ -0,0 +1,9 @@
+    socket = require"socket"
+    socket.unix = require"socket.unix"
+    u = assert(socket.unix.udp())
+    assert(u:bind("/tmp/foo"))
+    while 1 do
+		x, r = assert(u:receivefrom())
+		print(x, r)
+		assert(u:sendto(">" .. x, r))
+    end