ssh-agent add PKCS#11 support

[Alon Bar-Lev]

Hello,

PKCS#11 is a standard API interface that can be used in 
order to access cryptographic tokens. You can find the 
specification at 
http://www.rsasecurity.com/rsalabs/node.asp?id=2133, most 
smartcard and other cryptographic device vendors support 
PKCS#11, opensc also provides PKCS#11 interface.

I can easily make the scard.c, scard-opensc.c and 
ssh-agent.c support PKCS#11.

PKCS#11 is much more portable, standard, used standard than 
the current opensc implementation.

I just written the PKCS#11 support for the openvpn project, 
and I think openssh can also benefit from the same 
implementation.

Are you interested in merging PKCS#11 support? I don't won't 
to create a separate branch. After implementing the PKCS#11 
support you can drop the opensc code, users can use the 
opensc PKCS#11 provider in order to access their keys.

Does the current implementation of ssh-agent is the final 
one? I am asking this before I implement code that may be 
dramatically changed (For example, support X509 and PKIX).

Best Regards,
Alon Bar-Lev.