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

src/pkg/os/file_unix.go

@@ -152,7 +152,7 @@ func (f *File) readdir(n int) (fi []FileInfo, err error) {
 		if err == nil {
 			fi[i] = fip
 		} else {
-			fi[i] = &FileStat{name: filename}
+			fi[i] = &fileStat{name: filename}
 		}
 	}
 	return fi, err