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.

clients/cli/utils.c

@@ -390,7 +390,7 @@ nmc_terminal_show_progress (const char *str)
 	const char slashes[4] = {'|', '/', '-', '\\'};
 
 	nmc_terminal_erase_line ();
-	g_print ("%c %s", slashes[idx++], str ? str : "");
+	g_print ("%c %s", slashes[idx++], str ?: "");
 	fflush (stdout);
 	if (idx == 4)
 		idx = 0;