NETINFO cells, which are sent in every handshake, may contain
timestamps.  This patch adds an accessor for the timestamp in the
Netinfo messages, and teaches the tor-proto code how to compute the
minimum clock skew in the code.

The computation isn't terribly precise, but it doesn't need to be:
Tor should work fine if your clock is accurate to within a few
hours.

This patch also notes a Y2038 problem in the protocol: see
torspec#80.

Part of #405.