- make python2 and python3 compatible
- support Checkpoint create flags (requires recent libnm for GI
of flags)
- support adjust-rollback-timeout command
- print elapsed time in show output
Note that this changes API for checkpoint_create_async() in Python
via GIR. Previously it would require an integer argument, now a flags
argument. But this API is still unstable, it will be introduced with
1.12.