Revive Amiga port for NetHack 3.7

Update the Amiga Intuition window port (AMII/AMIV) for the 3.7
window_procs API. Key changes:

- Update all window function signatures for 3.7
- Add assembly trampolines for AmigaOS register-based callbacks
- Convert all K&R function definitions to C99
- Add cross-compilation build system (cross-pre1/pre2/post.370)
  using bebbo's m68k-amigaos-gcc with -noixemul -std=gnu17 -m68000
- Clipping fixes: viewport centering, simplified ScrollRaster,
  duplicate Ctrl-R suppression, glyph buffer invalidation
- Add menucolor support in menu rendering
- Move native txt2iff.c and xpm2iff.c to outdated/
- Add nethack.cnf and README.amiga

include/winprocs.h

@@ -13,8 +13,11 @@
 enum wp_ids { wp_tty = 1, wp_X11, wp_Qt, wp_mswin, wp_curses,
               wp_chainin, wp_chainout, wp_safestartup, wp_shim,
               wp_hup, wp_guistubs, wp_ttystubs,
+#if defined(AMIGA)
+              wp_amii, wp_amiv,
+#endif
 #ifdef OUTDATED_STUFF
-              wp_mac, wp_Gem, wp_Gnome, wp_amii, wp_amiv,
+              wp_mac, wp_Gem, wp_Gnome,
 #endif
               wp_trace	// XXX do we need this?  should chainin/out get an id? TBD
 };