Call fsync() on the configuration directories where appropriate.

Call fsync() on the configuration directories where appropriate.

Just ensuring individual config files are written atomically is good enough
to keep internal consistency, but we want to make sure directory metadata
is also synced to disk when returning from functions that expect the changes
to have been made permanent, such as meshlink_import() and
meshlink_blacklist(). Also do this when we create the initial directory
structure.

We already took care of syncing directory metadata when rotating keys for
encrypted storage.