Puppet::FileSystem.open / exclusive_open / replace_file should use Windows file system support for setting `mode`

While working on some test fixes for PUP-6188, it was noticed that the mode parameter being accepted mimics the behavior of the Ruby File APIs. These APIs are mostly broken on Windows and do weird things to calculate a mode based on Windows File attributes (like read-only, archive, reparse point, etc) instead of attempting to simulate based on actual ACL permissions (see code at https://github.com/ruby/ruby/blob/trunk/win32/win32.c#L5419-L5465). Therefore, the Ruby File.stat(path).mode will typically not roundtrip properly.

Puppet already works around Rubys mode behavior, and will patch stat and the stats mode when using the Puppet::FileSystem.stat or similar methods. However, when specifying a mode parameter explicitly to Puppet::FileSystem.open or exclusive_open, instead of using the Puppet helpers, the Ruby behavior is observed. This is probably not what we want, as it's mostly useless.

Some tests were added documenting this behavior in:
https://github.com/puppetlabs/puppet/commit/0a582a3c3a2aae03a68efaea0556e3d8f3c68b47

The proposal is to call the Windows helper when passing a mode in any of the Puppet::FileSystem APIs. This is technically a breaking change, even if its doubtful any existing code actually relies on this strange behavior. An audit of the forge should be done at the same time as an audit of the Puppet code. This is tentatively targeted at Puppet 5 to make the official break.