Add a new internal-autosync API to perform autosync.

Add a new internal-autosync API to perform autosync.

Instead of explicitly calling umount-all; sync, we add a daemon
function called internal-autosync which does the same.

Apart from slightly simplifying the process of closing the handle, the
main advantage is we can modify the daemon for the standalone case so
that internal-autosync does not do the umount-all operation.