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.

lib/repo.c

@@ -277,7 +277,7 @@ xbps_repo_get_pkg_plist(struct xbps_handle *xhp, xbps_dictionary_t pkgd,
 	if (url == NULL)
 		return NULL;
 
-	bpkgd = xbps_get_pkg_plist_from_binpkg(url, plist);
+	bpkgd = xbps_binpkg_get_plist(url, plist);
 	free(url);
 	return bpkgd;
 }