os: turn FileStat.Sys into a method on FileInfo

os: turn FileStat.Sys into a method on FileInfo

This reduces the overhead necessary to work with OS-specific
file details, hides the implementation of FileStat, and
preserves the implementation-specific nature of Sys.

Expressions such as:

  stat.(*os.FileInfo).Sys.(*syscall.Stat_t).Uid
  fi1.(*os.FileStat).SameFile(fi2.(*os.FileStat))

Are now spelled as::

  stat.Sys().(*syscall.Stat_t).Uid
  os.SameFile(fi1, fi2)

R=cw, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5448079