Give the kex selection list box a fixed height.

It's actually the limiting factor on how small the whole PuTTY
configuration dialog box can be: when KEX_MAX increased from 10 to 11
with the introduction of NTRU, the config box got taller. Now it's
back at 10.

config.c

@@ -2747,7 +2747,7 @@ void setup_config_box(struct controlbox *b, bool midsession,
             c = ctrl_draglist(s, "Algorithm selection policy:", 's',
                               HELPCTX(ssh_kexlist),
                               kexlist_handler, P(NULL));
-            c->listbox.height = KEX_MAX;
+            c->listbox.height = 10;
 #ifndef NO_GSSAPI
             ctrl_checkbox(s, "Attempt GSSAPI key exchange",
                           'k', HELPCTX(ssh_gssapi),