Amiga: header hygiene and palette-size constants

Add AMII_PALETTE_SIZE / AMIV_PALETTE_SIZE in amiconf.h to make the
actual populated portion of the init-map arrays explicit.  Drop the
redundant extern void exit() declaration.  Annotate Abort with
NORETURN in both amiconf.h and winproto.h; drop the duplicate Abort
declaration further down winproto.h.

Convert the bare-token "CLIPPING must be defined" assertion in
windefs.h into a real #error directive.

Comment in winext.h to disambiguate the three similarly named
amii*_init*map palette arrays.

sys/amiga/winproto.h

@@ -59,7 +59,7 @@ int amikbhit(void);
 int WindowGetchar(void);
 WETYPE WindowGetevent(void);
 void amii_cleanup(void);
-void Abort(long rc);
+void Abort(long rc) NORETURN;
 void CleanUp(void);
 void flush_glyph_buffer(struct Window *w);
 void amiga_print_glyph(winid window, int color_index, int glyph);
@@ -120,8 +120,6 @@ void amii_display_file(const char *fn, boolean complain);
 void SetBorder(struct Gadget *gd);
 /* malloc/free provided by stdlib.h */
 
-void Abort(long rc);
-
 win_request_info *amii_ctrl_nhwindow(winid, int, win_request_info *);
 
 /* amirip.c */