xbps-query(8: added --cat=FILE mode support.

- This mode prints to stdout the matching FILE stored in a binary package.
- ABI break: renamed xbps_get_pkg_plist_from_binpkg() xbps_binpkg_get_plist().
- Added xbps_binpkg_get_file() as a generic way to get pkg file contents.
- Removed useless comments from xbps_api_impl.h.

NEWS

@@ -1,5 +1,10 @@
 xbps-0.42 (???):
 
+ * xbps-query(8): added --cat=FILE mode to print FILE stored in a binary package
+   to stdout. This works exactly as the --files mode: if the matching binary package
+   is not available in the cachedir, xbps will establish a network connection
+   to the target repository to get the contents of FILE on the fly.
+
  * Get rid of repodata index-files, that contained all pkg files of a repository
    in the archive. That file was so huge that incresed the repository archive
    size by 8x. That means that `xbps-query -Ro` will now make N parallel connections