* improve `std.fs.AtomicFile` to use sendfile()
- also fix AtomicFile cleanup not destroying tmp files under some
error conditions
* improve `std.fs.updateFile` to take advantage of the new `makePath`
which no longer needs an Allocator.
* rename std.fs.makeDir to std.fs.makeDirAbsolute
* rename std.fs.Dir.makeDirC to std.fs.Dir.makeDirZ
* add std.fs.Dir.makeDirW and provide Windows implementation of
std.os.mkdirat. std.os.windows.CreateDirectory is now implemented
by calling ntdll, supports an optional root directory handle,
and returns an open directory handle. Its error set has a few more
errors in it.
* rename std.fs.Dir.changeTo to std.fs.Dir.setAsCwd
* fix std.fs.File.writevAll and related functions when len 0 iovecs
supplied.
* introduce `std.fs.File.writeFileAll`, exposing a convenient
cross-platform API on top of sendfile().
* `NoDevice` added to std.os.MakeDirError error set.
* std.os.fchdir gets a smaller error set.
* std.os.windows.CloseHandle is implemented with ntdll call rather than
kernel32.
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