os: deprecate os.SEEK_SET, os.SEEK_CUR, and os.SEEK_END

CL/19862 introduced the same set of constants to the io package.
We should steer users away from the os.SEEK* versions and towards
the io.Seek* versions.

Updates #6885

Change-Id: I96ec5be3ec3439e1295c937159dadaf1ebfb2737
Reviewed-on: https://go-review.googlesource.com/21540Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file.go

@@ -66,6 +66,8 @@ const (
 )
 
 // Seek whence values.
+//
+// Deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.
 const (
 	SEEK_SET int = 0 // seek relative to the origin of the file
 	SEEK_CUR int = 1 // seek relative to the current offset