agent: Fix for possible support of Cygwin OpenSSH.

* agent/command-ssh.c (start_command_handler_ssh): Use es_sysopen.

With new (not-yet-released) libgpg-error, gpg-agent should be able to
handle connection from Cygwin version of OpenSSH.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>