Add support for HTTP 1.1 If-Modified-Since behavior.

fetch(1) accepts a new argument -i <file> that if specified will cause
the file to be downloaded only if it is more recent than the mtime of
<file>.

libfetch(3) accepts the mtime in the url structure and a flag to
indicate when this behavior is desired.

PR: bin/87841
Submitted by: Jukka A. Ukkonen <jau@iki.fi> (partially)
Reviewed by: des, ru
MFC after: 3 weeks