int libssh2_agent_get_identity(LIBSSH2_AGENT *agent,
struct libssh2_agent_publickey **store,
struct libssh2_agent_publickey *prev);
store should point to a pointer that gets filled in to point to the public key data.
prev is a pointer to a previous 'struct libssh2_agent_publickey' as returned by a previous invoke of this function, or NULL to get the first entry in the internal collection.
Returns 1 if it reached the end of public keys.
Returns negative values for error