Refactored and better commented the Boost Build msvc toolset module as it grew to be quite large and lose its structure while even more features are being planned for it. No functional changes.
Organized content into: module inclusions, public rules, class definitions, local rules and finally startup code, in that order. Rules inside a single group ordered alphabetically. Mostly standardized global variable naming style. Extracted all toolset, flag & generator registrations into a single helper rule. Extracted all the msvc version auto-detection code into a single local helper rule. toolset.flag rule now referenced by its full name. Minor stylistic changes.
