"token_mode" add support for "script", which execute "token_script" to
get the password. Some token is not supported by OpenConnect natively,
e.g. "MobilePass" or "Softoken II" used in Cisco VPN
Signed-off-by: Gavin Ni <gisngy@gmail.com>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>