You can now reboot from Settings in iOS11 if the device is responsive and unlocked.

If it's locked or you just want to properly shutdown quickly, press and hold a volume button and the lock button to bring up the SOS mode, which includes the shut down slider.

If the device is unresponsive, it's much less obvious now: https://ios.gadgethacks.com/how-to/force-restart-iphone-x-wh... Volume up, then down, then press and hold the lock button.