Make sure the right element of the game-type dropdown starts off

Make sure the right element of the game-type dropdown starts off
selected. Previously we were leaving the first element on the list
selected, which is _usually_ right, but not right for Slant.

In the process of doing this, I've also reorganised to fix a crash
which shows up with non-configurable games (admittedly currently only
Nullgame :-) when we still try to call js_select_preset in spite of
not having any preset options to select.

[originally from svn r9794]