This means that, on the one hand, an absentminded press of Return
doesn't dismiss the entire CA config box, which would be pretty
annoying if you were half way through entering a load of fiddly stuff.
And on the other hand, you _can_ press Escape to dismiss the box,
which is less likely to happen by accident.
This allows you to load a CA public key from a disk file (in any
format acceptable to ppk_load_pub, which means OpenSSH one-line public
keys and also RFC4716 ones).
In the course of polishing up this dialog box, I'm going to want it to
actually do cryptographic things (such as checking validity of a
public key blob and printing its fingerprint), which means it will
need to link against SSH utility functions.
So I've moved the dialog-box setup and handling code out of config.c
into a new file in the ssh subdirectory and in the ssh library, where
those facilities will be conveniently available.
This also means that dialog-box setup code _won't_ be linked into
PuTTYtel or pterm (on either platform), so I've added a stub source
file to provide its entry-point function in those tools. Also,
provided a const bool to indicate whether that dialog is available,
which we use to decide whether to recognise that command-line option.
