Make HelpCtx a per-platform type, not an intorptr.

Partly, this just seems more sensible, since it may well vary per
platform beyond the ability of intorptr to specify. But more
immediately it means the definition of the HELPCTX macro doesn't have
to use the P() function from dialog.h, which isn't defined in any
circumstances outside the config subsystem. And I'm about to want to
put a help context well outside that subsystem.

utils/ctrlset_normalise.c

@@ -2,6 +2,7 @@
  * Helper function from the dialog.h mechanism.
  */
 
+#include "putty.h"
 #include "misc.h"
 #include "dialog.h"