New typedef 'dlgcontrol' wrapping 'union control'.

I'm about to change my mind about whether its top-level nature is
struct or union, and rather than change the key word 'union' to
'struct' at every point of use, it's nicer to just get rid of the
keyword completely. So it has a shiny new name.

defs.h

@@ -172,6 +172,7 @@ typedef struct NTRUKeyPair NTRUKeyPair;
 typedef struct NTRUEncodeSchedule NTRUEncodeSchedule;
 
 typedef struct dlgparam dlgparam;
+typedef union control dlgcontrol;
 
 typedef struct settings_w settings_w;
 typedef struct settings_r settings_r;