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/monst.c

@@ -3236,6 +3236,8 @@ struct permonst _mons2[] = {
 
 #ifndef SPLITMON_1
 
+void NDECL(monst_globals_init); /* in hack.h but we're using config.h */
+
 struct permonst mons[SIZE(mons_init)];
 
 void