Ensure disk caches and aufs read-write branch are emptied during emergency shutdown

Originally created by @intrigeri on #12428 (Redmine)

… so that they are overwritten by kernel memory poisoning. “Emergency” shutdown = when one unplugs the boot device.

This needs to be done in a way that’s as reliable as possible: in particular, the storage medium may host the persistent filesystem. If unmounting doesn’t work well enough (or just as an additional safeguard), we should probably echo 3 > /proc/sys/vm/drop_caches at some point during the shutdown process.

Writing an automated test for the “Tails with persistent volume unlocked” and “aufs read-write branch” usecases would help confirming it actually works. It probably requires implementing a /lib/systemd/system-shutdown/ hook that pauses for a while when debug=wipemem is passed on the kernel command line, so that we can dump memory after we’ve tried unmounting the filesystems.

Feature Branch: bugfix/12354-drop-kexec-memory-wipe

Parent Task: #12354 (closed)