sysroot: Add a try_lock() API

sysroot: Add a try_lock() API

The blocking locking API wasn't sufficient for use in the rpm-ostree
daemon; it really wants to know if the lock is held, then continue to
do other things (like service DBus requests), and get notification
when the lock is available.

We also add an async variant that can be called if the lock is not
available.

Implement a higher level "loop until lock is available" method in the
`ostree admin` commandline.