implement sendto()/recvfrom() equivalents.

download

this saves around 70us per received packet (which is around 50% of the
overall packet processing time).

other protocol won't work at the moment.

exception rather than the rule; we may as well keep things clean for other
protocols.

will enable us to cascade async operations, which is necessary in order to
properly support DNS.  (For example, an HTTP request may have to redirect
to a new location and will have to perform a new DNS lookup, so we can't
just rely on doing the name lookup at the time of parsing the initial
URL).

Anything other than HTTP is probably broken right now; I'll fix the others
up asap.