OpenPGP: Read some empty DOs from Gnuk.

In Gnuk, some empty DOs are returned as not exist, instead of existing with empty value.
So, we will consider them exist in driver.
PKCS15-OpenPGP: Do not show empty DO in pkcs15 emu_init.
PKCS15-OpenPGP: Allow to store data to pkcs15 data object.

Only one DO is supported now.
Do not do erase card on Gnuk (Gnuk does not support).
openpgp-tool: Correct function to delete key in Gnuk.
OpenPGP: Provide enough buffer to key generation on Gnuk.
pkcs15-openpgp: Change logging message.
Core: Fix bug with GET_RESPONSE (#147)

https://github.com/OpenSC/OpenSC/issues/147