Avoid launching a session from the Default Settings, even if they do

Avoid launching a session from the Default Settings, even if they do
represent a launchable session, unless the user can be construed to
have really meant it. This means:
 - starting up PuTTY when the Default Settings are launchable still
   brings up the config box, and you have to hit Open to actually
   launch that session
 - double-clicking on Default Settings from the config box will load
   them but not launch them.
On the other hand:
 - explicitly loading the Default Settings on the command line using
   `-load' _does_ still launch them.

[originally from svn r7265]