running_under_secureboot: update for efivarfs + new mokutil behavior

running_under_secureboot: update for efivarfs + new mokutil behavior

CONFIG_EFI_VARS is no longer available since
https://salsa.debian.org/kernel-team/linux/-/commit/20146398c4599147244ed3ffc54f38d07fb8dea3
(tagged initially as debian/5.10.1-1_exp1 + shipped with kernel package
5.10.1-1~exp1 and newer, incl. 5.10.0-12-amd64 as present in current
Debian/bullseye). Therefore the kernel module efivars is no longer
available on more recent Debian kernel systems, but efivarfs
needs to be used instead.

The behavior of mokutil also seems to have changed. On systems where
SecureBoot is available but not enabled, it outputs "SecureBoot
disabled", while no longer returning with an exit code other than 0.
On systems where Secure Boot isn't supported (e.g. in VirtualBox) it
reports "This system doesn't support Secure Boot" on stderr, with exit
code 255.

Verified with mokutil 0.3.0+1538710437.fb6250f-1

This work was funded by Grml-Forensic.