std: Push process stdio setup in std::sys

std: Push process stdio setup in std::sys

Most of this is platform-specific anyway, and we generally have to jump through
fewer hoops to do the equivalent operation on Windows. One benefit for Windows
today is that this new structure avoids an extra `DuplicateHandle` when creating
pipes. For Unix, however, the behavior should be the same.

Note that this is just a pure refactoring, no functionality was added or
removed.