net.nim: add support for Unix sockets

examples/unix_socket/client.nim

@@ -0,0 +1,6 @@
+import net
+
+let sock = newSocket(AF_UNIX, SOCK_STREAM, IPPROTO_IP)
+
+sock.connectUnix("sock")
+sock.send("hello\n")

examples/unix_socket/server.nim

@@ -0,0 +1,14 @@
+import net
+
+let sock = newSocket(AF_UNIX, SOCK_STREAM, IPPROTO_IP)
+sock.bindUnix("sock")
+sock.listen()
+
+while true:
+  var client = new(Socket)
+  sock.accept(client)
+  var output = ""
+  output.setLen 32
+  client.readLine(output)
+  echo "got ", output
+  client.close()