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/rpool.c

@@ -315,7 +315,7 @@ xbps_rpool_get_pkg_plist(struct xbps_handle *xhp,
 		errno = EINVAL;
 		goto out;
 	}
-	plistd = xbps_get_pkg_plist_from_binpkg(url, plistf);
+	plistd = xbps_binpkg_get_plist(url, plistf);
 	free(url);
 
 out: