2nd Patch - BIFF5 era XOR crypto

[Nick Lamb <njl98r ecs soton ac uk>]

Revised patch, still against latest 1.2 CVS since I'm fairly sure this
machine won't have the dependencies needed to build and test 1.3

This version deals with actual single-sheet BIFF crypto, which was not
previously working because the FILEPASS is in the different place. In
a sense this patch is a bug-fix, because there's no way the average user
knows if a file used XOR crypto or the later more secure stuff. I guess
it still won't make 1.2.x though :(

BTW If you have such files and can't find the password, I also announced
a Free Software version of those XL password finders you see advertised,
on my source code page http://www.ecs.soton.ac.uk/~njl98r/code/

Nick.