Add helper 'for_each_dive()' dive iterator

It's an easy thing to do, but the for-loop ends up being pretty ugly, so
hide it behind the macro.

It would be even prettier with one of the (few) useful C99 features:
local for-loop variables.  However, gcc needs special command line
options, and other compilers may not do it at all. So instead of doing

   #define for_each_dive(_x) \
      for (int _i = 0; ((_x) = get_dive(_i)) != NULL; _i++)

we require that the user declare the index iterator too, and the use
syntax becomes

   for_each_dive(idx, dive) {
	... use idx/dive here ...
   }

And hey, maybe somebody actually will want to use the index, so maybe
that's not all bad.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

divelist.c

@@ -85,7 +85,7 @@ void dump_selection(void)
 	struct dive *dive;
 
 	printf("currently selected are %d dives:", amount_selected);
-	for (i = 0; (dive = get_dive(i)) != NULL; i++) {
+	for_each_dive(i, dive) {
 		if (dive->selected)
 			printf(" %d", i);
 	}