Avoid reliance on /dev/fd/XXX (broken on BSD)

   Fix a regression introduced with verison 1.2.0.  The new download
   procedure relies on libmrss accessing the downloaded feed via
   /dev/fd/<fileno>.  This doesn't work on FreeBSD and OpenBSD, where
   these files are implemented as character devices.

   Thanks to Anton Lindqvist (anton at basename.se) for spotting.