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).
56 jobs for lr/nmcli-checkpoint in 45 minutes and 14 seconds (queued for 3 seconds)
Name Stage Failure
failed
check-patch Test
src/nmcli/devices.c:5164: Don't use "unsigned int" but just use "unsigned":
> long unsigned int timeout = 15;

src/nmcli/devices.c:5195: Don't use strcmp/g_strcmp0 unless you need to sort. Consider nm_streq()/nm_streq0(),NM_IN_STRSET() for testing equality:
> if (strcmp(*argv, "--") == 0) {

The patch does not validate.
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1