The libnm API fir checkpoints was only introduced with 1.11. It
is not yet stable, so there is still time to adjust it. Note that
this changes API/ABI of the development branch.

Changes:

- we only add async variants of the checkpoint functions. I believe
  that synchronous D-Bus methods are fundamentally flawed, because
  they mess up the ordering of events.
  Rename the async functions by removing the "_async" suffix. This
  matches glib style, for which the async form is also not specially
  marked.

- for function that refere to a particular checkpoint (rollback and
  destroy), accept the D-Bus path as string, instead of an NMCheckpoint
  instance. This form is more flexible, because it allows to use
  the function without having a NMCheckpoint instance at hand. On the
  other hand, if one has a NMCheckpoint instance, he can trivially
  obtain the path to make the call.

- 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

We need common operations from the python scripts.
For example, to print the boot-time.

Move such utils to a separate nmex.py file ("ex" for
example). This file should contain helper functions that
are pure python (or, if the have requirements, load them
only on demand, so that examples that need those have
additional dependencies). It should also be simple to extract
individual helpers from nmex, so that the user can take an
example, merge parts of nmex.py in, and modify it to his needs.

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.

We commonly only allow tabs at the beginning of a line, not
afterwards. The reason for this style is so that the code
looks formated right with tabstop=4 and tabstop=8.

The example script touches the stable-id of a connection.
It does so blocking autoconnect, and was originally written
to test that functionality.

https://bugzilla.gnome.org/show_bug.cgi?id=791121

Show how to load and use the VPN plugins from python.