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.
Change check for RTC to be configure based.

Previously I had disabled it with #ifdef __linux__, but that's not
very good autoconf style.
Describe what an HTTP date is
Don't log raw dates, in case they contain junk.
Document magic "1900" value
In verbose mode, display TLS time as well as HTTPS time
Update manpage with http option
Make user-agent a configuration option
Add a changelog
Restrict loggable date fields even harder.
Merge pull request #121 from nmathewson/http_date

Rudimentary support for HTTP Date headers