bin/fsck: Make ref binding verification optional

bin/fsck: Make ref binding verification optional

Today the rpm-ostree test suite uses `refs --create` to save
commits.  I think this is a legitimate use case, and other
people may be doing something similar.

On the other hand, I think we should probably be changing the rpm-ostree test
suite to create "unbound" commits. But let's be maximially compatible here since
we hit a real-world case where something needed to change.

Closes: #1379
Approved by: pwithnall