Add check for switcheroo functionality when multiple GPUs are detected

Some Linux distributions do not enable switcheroo-control by default, despite it being part of most system installations these days. This implements a simple check to warn the user of degraded functionality if switcherooctl exists but returns no GPU list (as is the case when the service doesn't run).

For more information, see https://man.archlinux.org/man/switcherooctl.1.en