ssh-agent support

Hi!

Daiki Ueno <ueno_at_unixuser.org> writes:

> If it looks OK, I will try to implement it this weekend.

The task took longer than I expected ;-)
The attached is the initial implementation.

Notes:

- The API changed from the proposal after the manner of the knownhost API.
  Now the following 7 new functions added. See the
  example/simple/ssh2_agent.c for the usage.

 LIBSSH2_AGENT *libssh2_agent_init(LIBSSH2_SESSION *session);
 int libssh2_agent_connect(LIBSSH2_AGENT *agent);
 int libssh2_agent_list_identities(LIBSSH2_AGENT *agent);
 int libssh2_agent_get_identity(LIBSSH2_AGENT *agent,
                                struct libssh2_agent_publickey **store,
                                struct libssh2_agent_publickey *prev);
 int libssh2_agent_userauth(LIBSSH2_AGENT *agent,
                            const char *username,
                            struct libssh2_agent_publickey *identity);
 int libssh2_agent_disconnect(LIBSSH2_AGENT *agent);
 int libssh2_agent_free(LIBSSH2_AGENT *agent);

- libssh2_agent_{add,del}_identity() could also be added. It will be
  useful for testing, but it will require extension to the internal
  hostkey interface to encode DSA/RSA parameters in the agent protocol.

- Pageant support is not yet implemented but it should be easy if I take
  some code from the Putty source (putty/windows/winpgnt.c). Is that
  OK? The Putty License is the MIT License.

I would appreciate any comments.

Regards,

-- 
Daiki Ueno