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.

Add endpoint for deleting encryption key

Closes #6

See merge request !15

# Conflicts:
#   src/cryptops-api.c

When deleting an encryption key, check if there is at least one other
active keyslot, and do not delete if there isn't.

Resolve "Makefile does not track c file changes"

Closes #22

See merge request !16

Add endpoint for adding encryption key

Closes #5

See merge request !14

also move a comment in deploy.sh to make it more logical

Replace the very similar encryption_keys_post and encryption_keys_put
callback functions by a single handler function, with a boolean flag to
separate the two variants.

Add endpoint for listing encryption keys

Closes #4

See merge request !13

Resolve "Delete ssh-key endpoint"

Closes #9

See merge request !12

Add endpoint for removing encryption

Closes #10

See merge request !11