To an alias address, or in a multihoming environment, the internal dgram
server may respond from a different address than the destination address
sent by the client.
To solve this problem, I introduce sendfromto() and recvfromto() so that
the server can reply from the correct address, i.e., the destination address
used in the request.

This change also has the aspect of a reference implementation using the
RECVDSTADDR and PKTINFO socket options.

(ryo)