Regretfully remove my trickery with a hidden <option> element inside

Regretfully remove my trickery with a hidden <option> element inside
the game-type <select>, since IE turns out to ignore display:none on
options. Oh well.

Instead I now do a more transparent thing: when custom game params are
in use, there's a "Custom" option selected in the dropdown, and a
separate 'Re-customise' option which brings the config box back up.
When an ordinary preset is selected, the Custom option is missing, and
there's just a 'Customise'.

In the process I've tinkered a bit to arrange that the custom 'preset'
is always represented by a negative number rather than one past the
last real preset; that seems more consistent overall.

[originally from svn r9811]