support datagram unix domain sockets

test/unixudpclnt.lua

@@ -0,0 +1,9 @@
+socket = require"socket"
+socket.unix = require"socket.unix"
+c = assert(socket.unix.udp())
+c:bind("/tmp/bar")
+while 1 do
+    local l = io.read("*l")
+    assert(c:sendto(l, "/tmp/foo"))
+	print(assert(c:receivefrom()))
+end