Had been checking an uninitialized length field if it is an exact
multiple of the block length.

- Fix an off-by-one error in ISO7816 unpadding.
- Unpadding errors are not assertion failures, handle the error.
- Check all the requirements of the padding scheme when unpadding.

due to having to restart many varnishd instances.

The size of the tests has necessitated a hard-wired setting for the size
of libgcrypt secure memory; a better way to handle lib initialization is
on the to-do list.

enable_sync and cbc_mac flags are removed.

* IV and CTR are set for the en-/decrypt operations, not in the constructor.
  This means that the crypto handle must be copied to the stack during the
  en-/decrypt calls, because setting IV/CTR changes its internal state.
* Add code for padding (not yet tested).

00basic.vtc tests the gcrypt_version() function, which implicitly tests
initialization of libgcrypt.