fileutils: Add API to fstat() a stream

Will be used by ostree.  Since libgsystem will return raw
GUnix{Input,Output}Stream, we need a way to fstat() them.  Maybe we
should also have an API to synthesize a #GFileInfo, but that's hard
without duplicating all the complex stuff in GIO.

https://bugzilla.gnome.org/show_bug.cgi?id=706031