A more detailed sketch of nonce handling

This ought to work except that (a) we're not actually sending or reading
in the nonce, (b) even if we were, increment_nonce isn't doing anything,
so the memcmp will fail to accept the nonce.
Make udp_read/udp_write handle nonces

Instead of doing more buffer manipulation, we use recvmsg and sendmsg to
read and write multiple buffers. One is the nonce (or, perhaps someday,
a message header) and the other is the data. This works, save for the
nonce check.

(The other changes are all s/crypto_box_NONCEBYTES/NONCEBYTES/)
Propagate msg_namelen change to the caller after recvmsg
Implement increment_nonce (by 2, for now)