Check ServerVersion first when retrieving Darwin product version.

This is the order the operating system checks in.

Change-Id: I5577c09867eb8c034bf1d47daba9d69b907ba62d
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Define hostOSBuildVersion on Windows as well.

This mirrors CoreFoundation's behavior (and makes sense given the name).

Change-Id: I4519978cff507f585378469cf535c2b815172fd4
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>