all: use the elvis operator wherever possible

Coccinelle:

  @@
  expression a, b;
  @@
  -a ? a : b
  +a ?: b

Applied with:

  spatch --sp-file ternary.cocci --in-place --smpl-spacing --dir .

With some manual adjustments on spots that Cocci didn't catch for
reasons unknown.

Thanks to the marvelous effort of the GNU compiler developer we can now
spare a couple of bits that could be used for more important things,
like this commit message. Standards commitees yet have to catch up.

src/platform/nm-linux-platform.c

@@ -6980,7 +6980,7 @@ handle_udev_event (NMUdevClient *udev_client,
 	seqnum = udev_device_get_seqnum (udevice);
 	_LOGD ("UDEV event: action '%s' subsys '%s' device '%s' (%s); seqnum=%" G_GUINT64_FORMAT,
 	        action, subsys, udev_device_get_sysname (udevice),
-	        ifindex ? ifindex : "unknown", seqnum);
+	        ifindex ?: "unknown", seqnum);
 
 	if (NM_IN_STRSET (action, "add", "move"))
 		udev_device_added (platform, udevice);