Init an ssh-agent handle. Returns the handle to an internal
representation of an ssh-agent connection. After the successful
initialization, an application can call libssh2_agent_connect(3)
to connect to a running ssh-agent.
Call libssh2_agent_free(3) to free the handle again after you're
doing using it.
RETURN VALUE
Returns a handle pointer or NULL if something went wrong. The returned handle
is used as input to all other ssh-agent related functions libssh2 provides.