checkout: Add --fsync=false

checkout: Add --fsync=false

Some use cases for checkouts don't need to fsync during checkout.
Installer programs for example will just do a global fsync at the end.

In the future, the default "ostree admin" core could also be
rearchitected to only do a transaction commit right before reboot, and
do the fsync then.

https://bugzilla.gnome.org/show_bug.cgi?id=742482