Patching to make compatible with appengine/socket.

The interface conversion from socket.Conn to net.UDPConn fails, but
since you can use a generic Read(b) in place of ReadFromUDP(b) it makes
sense to git rid of the conversion to net.UDPConn.  This change allows
the use of the client library for UDP DNS lookup using the
appengine/socket package's Conn.

For the same reasons the interface conversions for TCP connections were
also removed.