add SSL client certificate password prompt support

irssi doesn't currently prompt you for a passphrase so lets enable this if the user didn't supply one and a client certificate was used.
Core should be ui-agnostic, so this is not the place to directly prompt for the password.
So this needs some indirection, and then, if possible (but might be hard because it's asynchronous) use the same mechanism that /oper uses to prompt for the password (keyboard_entry_redirect,...).