If a system administrator happens to type `ostree admin upgrade`
multiple times, currently that will lead to a potentially corrupted
system.
I originally attempted to do locking *internally* in `libostree`, but
that didn't work out because currently a number of the commands
perform multi-step operations that all need to be serialized. All of
the current code in `ostree admin deploy` is an example.
Therefore, allow callers to perform locking, as most of the higher
level logic is presently implemented there.
At some point, we can revisit having internal locking, but it will be
difficult. A more likely approach would be similar to Java's approach
with concurrency on iterators - a "fail fast" method.
Colin Walters