Introduce a new flag NM_CHECKPOINT_CREATE_FLAG_ALLOW_OVERLAPPING
that allows the creation of overlapping checkpoints. Before, and
by default, checkpoints that reference a same device conflict,
and creating such a checkpoint failed.

Now, allow this. But during rollback automatically destroy all
overlapping checkpoints that were created after the checkpoint
that is about to rollback.

With this, you can create a series of checkpoints, and rollback them
individually. With the restriction, that if you once rolled back to an
older checkpoint, you no longer can roll"forward" to a younger one.

What this implies and what is new here, is that the checkpoint might be
automatically destroyed by NetworkManager before the timeout expires. When
the user later would try to manually destroy/rollback such a checkpoint, it
would fail because the checkpoint no longer exists.