allowing the user to choose the name of the save file and specify it
on the command line when restoring. It also eliminates a buffer
overrun in determining the path to the save file, and any particular
arbitrary limit on the name length. In the name of a tidier home
directory, the default name is changed from "Bstar" to ".Bstar".
Patch supplied in PR 8085 by Joseph Myers <jsm28@cam.ac.uk>
Minor modification (s/startup/filename/ in initialize()) by me.
