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.