Update the docs to describe what affects how much secure memory may
be needed, and also to describe the limitations imposed by
PTHREAD_KEYS_MAX.

The previous implementation was not thread safe.
Now use pthread keys to create a handle for each object in each
thread, and then retrieve them in the same thread.

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.