libsa: Consolidate tftp sendrecv into net.c sendrecv

bootp/arp/rarp/rpc all use the sendrecv implementation in net.c. tftp has
its own implementation because it passes an extra parameter into the recv
callback for the received payload type to be held.

These sendrecv implementations are otherwise equivalent, so consolidate
them. The other users of sendrecv won't be using the extra argument to recv,
but this gives us only one place to worry about respecting timeouts and one
consistent timeout behavior.

Tested by: sbruno
Reviewed by: sbruno, tsoome
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14373