Lua: nhcore script with function callbacks

Adds possible callbacks for "start_new_game", "restore_old_game",
"moveloop_turn", and "game_exit" which when defined, will be called
from core code at the appropriate time.

Adds lua hooks for dump_fmtstr (only if DUMPLOG), dnum_name, u.moves,
u.uhave_amulet, and u.depth.

include/decl.h

@@ -995,6 +995,9 @@ struct instance_globals {
     int lusername_size;
 #endif
 
+    /* nhlua.c */
+    genericptr_t luacore; /* lua_State * */
+
     /* o_init.c */
     short disco[NUM_OBJECTS];