Backend: Fixed potential double free on exit.

The code that freed registered fd boxes on termination of a backend
did not invalidate the reference, this may have caused a reference
underflow for fds that still were registered in the now dead backend.

We now use the proper unhook code to de-register the fd boxes.
Use correct Alert for the errors.
#pragma strict_types
#pragma strict_types
Free up the symbol DEBUG.
Revert most of "Use correct Alert for the errors."

We need to avoid the circular compilation-time dependency here,
else the dumping will fail.

This reverts commit 33d666a15c27b9bef23f5dd8b513043f70165097.
Crypto.DH: Fixed some warnings.
Crypto.DH: Fixed some more warnings.
Nettle: CCM mode now actually works as intended.

Fixes multiple bugs in CCM mode (mainly with the digest).

Also adds the test vectors for AES-CCM from RFC 3610.