Improved download procedure

   So far, Crossbow relied on the mrss_parse_url call for parsing remote
   feeds.  Now Crossbow directly uses libcurl, providing libmrss with a
   locally downloaded file instead.

   This modification allows Crossbow to be in control of the HTTP/HTTPS
   calls configuration, opening up for future developments.

   The crossbow-fetch(1) command now supports parallel download jobs (-j).

   WARNING: this modification requires libcurl >= 7.66.0, which is not
   currenlty available in some systems (e.g. Debian stable).  A backport
   is feasible, so contact the developer (dacav at fastmail dot com) in
   case of need.

Minor build system improvements

Minor code improvements