support datagram unix domain sockets

test/unixtcpclnt.lua

@@ -0,0 +1,8 @@
+socket = require"socket"
+socket.unix = require"socket.unix"
+c = assert(socket.unix.tcp())
+assert(c:connect("/tmp/foo"))
+while 1 do
+    local l = io.read()
+    assert(c:send(l .. "\n"))
+end