Allow for alternative Blorb file at command line for curses, sdl, and dumb.

Allow for alternative Blorb file at command line for curses, sdl, and dumb.

Doing this for the SDL interface is proved a bit troublesome because
it's not very clear where stuff is being set and where not.  After the
audio overhaul release, I want to overhaul the SDL interface code to
make it as closely resemble as feasible that of the curses interface.