Summary:
Move the `UserInfo` code from `fuse/privhelper` to `utils`, and also unify the
POSIX and Windows implementations of this class.
This code was originally put in the `fuse/privhelper` directory since it was
written at the same time as the privhelper. However, it's really a
lower-level library that doesn't depend on FUSE or any of the other code in
the `fuse/` subdirectory.
Reviewed By: wez
Differential Revision: D21296594
fbshipit-source-id: f58682f6ce86bba0328472c491bb4c0dc3370319
