Another bunch of fixes and improvements

Summary of changes:
* one memory bug fixed (detected by Valgrind, confirmed by code inspection)
* HTML documentation cleaned up (validated using nsgmls and tidy)
* delay when shutting down BToolkit fixed
* instructions and configuration file for enabling anti-aliased fonts in BMotif
* several unreferenced global variables removed
* compatibility fixes for Solaris 10 (both 32- and 64-bit x86 works now)
* code cleanups

Now the BToolkit seems usable, at least for learning B with "The B-Method: An introduction" by Steve Schneider.

I still have several ideas for improvements, but the BToolkit code is quite difficult to maintain and implementing them is rather time consuming.