sysfs: adds FS.Stat and companions in platform (#1140)

This centralizes filestat logic by making our own `Stat_t` similar to
`syscall.Stat_t`. This exposes utilities in the platform package and
adds a new function `FS.Stat` which avoids having to use `fs.File` to
get the same info. Doing so at the FS abstraction allows us to optimize
how it is implemented internally using portable means (e.g.
`os.StatFile`) or OS-specific means where necessary, e.g. in windows.

This also ensures `platform.OpenFile` returns syscall.Errno and
centralizes error checking with a new `require.EqualErrno` test.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/fstest/fstest.go

@@ -100,17 +100,16 @@ func WriteTestFiles(tmpDir string) (err error) {
 			}
 
 			// os.Stat uses GetFileInformationByHandle internally.
-			stat, err := os.Stat(path)
-			if err != nil {
+			var stat platform.Stat_t
+			if err = platform.Stat(path, &stat); err != nil {
 				return err
 			}
-			if stat.ModTime() == info.ModTime() {
+			if stat.Mtim == info.ModTime().UnixNano() {
 				return nil // synced!
 			}
 
 			// Otherwise, we need to sync the timestamps.
-			atimeNsec, mtimeNsec, _ := platform.StatTimes(stat)
-			return os.Chtimes(path, time.Unix(0, atimeNsec), time.Unix(0, mtimeNsec))
+			return os.Chtimes(path, time.Unix(0, stat.Atim), time.Unix(0, stat.Mtim))
 		})
 	}
 	return