support datagram unix domain sockets

test/unixtcpsrvr.lua

@@ -0,0 +1,9 @@
+    socket = require"socket"
+    socket.unix = require"socket.unix"
+    u = assert(socket.unix.tcp())
+    assert(u:bind("/tmp/foo"))
+    assert(u:listen())
+    c = assert(u:accept())
+    while 1 do
+        print(assert(c:receive()))
+    end