io::FileStat needs to be less platform-dependent

[alexcrichton]

Right now there's a few bits of FileStat which should get massaged:

• mode needs to fit with the general solution of file permissions used in file I/O. I'm prototyping something right now, but I"m not happy how it's turning up. Regardless, the second argument to mkdir should be the same type as this field.
• created/modified/accessed - The documentation says that they're platform dependent. They're supposed to not be.
• inode/device - Are these really platform independent? If so, I don't know much about them and it would be nice to have some documentation
• is_dir/is_file - why are these explicit? What about pipes, sockets, links, etc.

Nominating for backcompat-libs 1.0 - it'd be a little embarassing to ship with this.

[alexcrichton]

Un-nominating, parenting to a nominated metabug.