Add remotes-config-dir to OstreeRepo

This allows you to replace the default
$sysroot/$sysconfdir/ostree/repos.d string value, and to use a similar
feature for repos that are not the system repo.

In particular, this allows us to support /etc/xdg-app/remotes.d for
xdg-app.

Closes: #247
Approved by: cgwalters

src/libostree/ostree-repo-private.h

@@ -63,6 +63,7 @@ struct OstreeRepo {
   int uncompressed_objects_dir_fd;
   GFile *config_file;
   GFile *sysroot_dir;
+  char *remotes_config_dir;
 
   GFile *transaction_lock_path;
   GHashTable *txn_refs;