option fixes and missing prototypes

I added -Wmissing-prototypes to my CFLAGS and got a bunch of warnings.
This fixes the core ones (there are more for X11 that I haven't looked
at yet).  While fixing these, I discovered a few option processing
issues:  the non-Amiga 'altmeta' should be settable while the game is
in progress (not sure about the Amiga variation so left that as-is),
'altmeta' and 'menucolor' are booleans so shouldn't have had optfn_XXX
functions; 'MACgraphics' and 'subkeyvalue' were conditionally defined
differently in options.c than in optlist.h.

src/objects.c

@@ -1170,6 +1170,8 @@ OBJECT(OBJ(None, None),
 /* clang-format on */
 /* *INDENT-ON* */
 
+void NDECL(objects_globals_init); /* in hack.h but we're using config.h */
+
 struct objdescr obj_descr[SIZE(obj_descr_init)];
 struct objclass objects[SIZE(obj_init)];