other protocol won't work at the moment.

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.

order so we can just use buffer->fill.

Remove the now-totally-obsolete sockaddr_in field from tcp.h.

transition away from uIP.

Prepared ipv4.c for transition away from uIP.

closed() method with a reason code.

connection and so legitimately ignores acked() and senddata().