First step towards parallel transmit in UDP: if neither a specific
source or a specific destination address is requested as part of a send
on a UDP socket, read lock the inpcb rather than write lock it. This
will allow fully parallel transmit down to the IP layer when sending
simultaneously from multiple threads on a connected UDP socket.

Parallel transmit for more complex cases, such as when sendto(2) is
invoked with an address and there's already a local binding, will
follow.

MFC after: 1 month