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.

src/decl.c

@@ -517,6 +517,9 @@ const struct instance_globals g_init = {
     MAX_LAN_USERNAME, /* lusername_size */
 #endif /* MAX_LAN_USERNAME */
 
+    /* nhlua.c */
+    UNDEFINED_VALUE, /* luacore */
+
     /* o_init.c */
     DUMMY, /* disco */