Add in_lua flag

In preparation of making config errors available to lua scripts,
instead outputting them via pline.

include/extern.h

@@ -1646,6 +1646,7 @@ extern int l_obj_register(lua_State *);
 
 #if !defined(CROSSCOMPILE) || defined(CROSSCOMPILE_TARGET)
 extern lua_State * nhl_init(void);
+extern void nhl_done(lua_State *);
 extern boolean nhl_loadlua(lua_State *, const char *);
 extern boolean load_lua(const char *);
 extern void nhl_error(lua_State *, const char *) NORETURN;