- backport forgotten Valid-Until patch from the obsolete experimental

  - backport forgotten Valid-Until patch from the obsolete experimental
    branch to prevent replay attacks better, thanks to Thomas Viehmann
    for the initial patch! (Closes: #499897)
* doc/apt.conf.5.xml:
  - document the new Valid-Until related options
* apt-pkg/contrib/strutl.cc:
  - split StrToTime() into HTTP1.1 and FTP date parser methods and
    use strptime() instead of some self-made scanf mangling
  - use the portable timegm shown in his manpage instead of a strange
    looking code copycat from wget
* ftparchive/writer.cc:
  - add ValidTime option to generate a Valid-Until header in Release file