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.
This seems wrong to me: there shouldn't be anything special about the window with refnum 1.
It might be the window that most people use as their "status" window, but that's just the default, should not be a hard-coded assumption.
If you don't want the password prompt to appear "anywhere", then what you should probably do is look for the window where messages for that server (with MSGLEVEL_CRAP maybe?) would go to.