bsd-user: Prepare for future upstream of system calls. This series of patches does three things. First, it starts to give up on the idea that you can run FooBSD binaries on BarBSD. They are too different to make that happen any time soon, though I've kept the support for Net/OpenBSD, even though they haven't built. We'll need a lot of work to make that happen, though, and I need to simplify to get things upstream. Second, it starts to move some of the ifdef trees into target.h. Third, it starts to upstream bsd-file.h, but the remainder of the file in the bsd-user fork had some issues that will be resolved before next quarter's update.