Add a checkout option to skip fsync

Add a checkout option to skip fsync

This is a better followup to dc9239dd7b09ef5e104309b4dbf0e136889da274
since I wanted to do fsync-less checkouts in rpm-ostree too, and
replicating the "turn off fsync temporarily" was in retrospect just a
hack.

We can simply add a boolean to the checkout options.

https://github.com/GNOME/ostree/pull/172