io: define SeekStart, SeekCurrent, SeekEnd constants for use with Seeker

Fixes #6885

Change-Id: I6907958186f6a2427da1ad2f6c20bd5d7bf7a3f9
Reviewed-on: https://go-review.googlesource.com/19862Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/io/io.go

@@ -16,6 +16,13 @@ import (
 	"errors"
 )
 
+// Seek whence values.
+const (
+	SeekStart   = 0 // seek relative to the origin of the file
+	SeekCurrent = 1 // seek relative to the current offset
+	SeekEnd     = 2 // seek relative to the end
+)
+
 // ErrShortWrite means that a write accepted fewer bytes than requested
 // but failed to return an explicit error.
 var ErrShortWrite = errors.New("short write")