- Back to timegm() using GnuPG's replacement if necessary.

- Back to timegm() using GnuPG's replacement if necessary.
  Using mktime() and localtime() could add a on hour offset if
  the randomize factor was big enough to lead to a summer/wintertime
  switch.

- Removed now-useless Privoxy 3.0.3 compatibility glue.

- Moved randomization code into pick_from_range().

- Changed parse_header_time definition.
  time_t isn't guaranteed to be signed and
  if it isn't, -1 isn't available as error code.
  Changed some variable types in client_if_modified_since()
  because of the same reason.