tools/build/v2/build-system.jam: Added support for toolset
autoconfiguration, both from the command-line, with toolset=xxx or --toolset=xxx, and by default when no tooset is specified.
tools/build/v2/build/feature.jam: added public is-subvalue rule and
factored out guts into local rule.
tools/build/index.html: changed redirect to point at v2 docs