(buil, drnuke_const, opt_DRNUKE, show, show_nuke_build)

(buil, drnuke_const, opt_DRNUKE, show, show_nuke_build)
(show_nuke_capab, vers, xdvisible, build.t, Options.t):
Remove opt_DRNUKE and replace with the following check
drnuke_const > MIN_DRNUKE_CONST.
Set MIN_DRNUKE_CONST to 0.001 to prevent issues with tiny values.
Change the default drnuke_const to 0.0 because in the stock game
opt_DRNUKE was disabled.