First sketch of security considerations

Closes #7

See merge request open/cryptops!2

start documentation in sphinx

Closes #1

See merge request !1

Run notification hook on startup

Closes #26

See merge request !22

Free memory after use

Closes #21

See merge request !21

# Conflicts:
#   src/api/encryption_init_post.c

Make device paths runtime configurable

Closes #24

See merge request !19

Add endpoint for selfdestruct

Closes #23

See merge request !18

On selfdestruct, first destroy all active keyslots through the
cryptsetup library, before zeroing the luks header.

Also take a factor two safety margin when zeroing the luks header,
and halt the machine (instead of rebooting) after selfdestruct.