Add libssh2_agent_sign() to allow sign any message

Sometimes user needs to sign some data on the remote host with his private ssh key which is located on the local host.

The only way to do this is to use ssh agent socket with common protocol.

 

Libssh2 contains function for such scenario but it could be used only inside of libssh2.

Is there any barrier to add such feature to public interface?

 

This patch adds proxy function for agent_sign() from agent.c

 

Could you pay attention to it please?

 

Thank you,

Igor Klevanets