Here's a patch that fills st_dev, and while we're at it st_rdev (which is the same value).

I've moved the implementation of samefile/sameopenfile/samestat from Lib/posixpath.py over to Lib/genericpath.py and then removed the implementation from Lib/ntpath.py, so those functions are now common. The same goes for tests - I've rearranged tests towards test_genericpath. I also removed _getfileinformation from Modules/posixmodule.c because it's no longer being used.