Added an `interactive' flag to new_game_desc(), which toggles Mines

between on the one hand generating indeterminate game descriptions
awaiting the initial click, and on the other hand generating
concrete ones which have had their initial click. This makes `mines
--generate' do something useful.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5869 cda61777-01e9-0310-a592-d414129be87e