nmcli/devices: add "checkpoint" command

This is an interface to the Checkpoint/Restore functionality that's
available for quite some time. It runs a command with a checkpoint taken
and rolls back unless success is confirmed before the checkpoint times
out:

  $ nmcli dev checkpoint eth0 -- nmcli dev dis eth0
  Device 'eth0' successfully disconnected.
  Type "Yes" to commit the changes: No
  Checkpoint was removed.

The details about how it's used are documented in nmcli(1) and
nmcli-examples(7).