Add boot.conf(8) 'mach idle [secs]' to halt at idle passphrase prompts
Enable users to power down their machines if there was no input after N
seconds during disk descryption.
Motivation is to save battery and prevent pocket heaters when notebooks
unhibernate (e.g. lid accidentially opened) and sit at "Passphrase: ".
Only available on efi(4) systems as the timeout is saved as EFI variable;
mostly because that's trivial to do, but also because we lack a better
mechanism to configure that and persist such data without the root disk.
Discussed with many, starting at h2k23
OK Tests gnezdo