Add os-version hook to system-info which returns f on every platform except windows...

Add os-version hook to system-info which returns f on every platform except windows, where it returns { 5 1 } for xp and { 6 1 } for win7.  Eventually do more here, like identify Snow Leopard, Lion, etc, but it's good enough to make ping tests pass.
Move system-info platform loader into system-info.backend
Add win7? and winxp? words to system-info, clean up ping-tests with new word
Merge system-info.backend into system-info. Will throw an exception when loading on *bsd now