Move host CA config box out into its own source file.

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.

putty.h

@@ -2589,6 +2589,7 @@ void setup_ca_config_box(struct controlbox *b);
 
 /* Platforms provide this to be called from config.c */
 void show_ca_config_box(dlgparam *dlg);
+extern const bool has_ca_config_box; /* false if, e.g., we're PuTTYtel */
 
 /* Visible outside config.c so that platforms can use it to recognise
  * the proxy type control */