Reset the NVRAM (Intel Mac)

Fixes some sound, resolution and startup disk issues.

1. Shut down the Mac.
2. Turn it on and immediately hold ⌥ + ⌘ + P + R for about 20 seconds.
3. Release: the Mac restarts. Adjust volume, resolution and startup disk if needed.
4. Apple silicon Macs have no such operation: the NVRAM is checked automatically at every startup, a simple restart is enough.