Rudimentary support for HTTP Date headers

Since I'm going on a personal crusade to kill off gmt_unix_time, I
should provide an alternative.  That alternative can be the Date
header from HTTP -- unlike gmt_unix_time, the Date header is required
by the RFC to actually be an accurate clock-like clock, and nobody is
trying to get rid of it.

This code is pretty hack-ish and does some nonportable stuff, like
using memmem() and timegm().  It's not super-tolerant of
non-standards-compliant HTTP servers.  I hope I didn't make any
pointer mistakes.

Please let me know if you think this is a promising direction.