Add shutdown hook interface to Lua API

Scripts can call minetest.register_on_shutdown() to register a
shutdown hook.

Document that minetest.register_on_shutdown() callbacks may not be run

If the server crashes, it is unlikely that callbacks registered using
minetest.register_on_shutdown() will be called.

src/scriptapi.h

@@ -55,6 +55,9 @@ void scriptapi_environment_step(lua_State *L, float dtime);
 void scriptapi_environment_on_generated(lua_State *L, v3s16 minp, v3s16 maxp,
 		u32 blockseed);
 
+/* server */
+void scriptapi_on_shutdown(lua_State *L);
+
 /* misc */
 void scriptapi_on_newplayer(lua_State *L, ServerActiveObject *player);
 void scriptapi_on_dieplayer(lua_State *L, ServerActiveObject *player);