CL/19862 (f79b50b8) recently introduced the constants
SeekStart, SeekCurrent, and SeekEnd to the io package. We should use these constants
consistently throughout the code base.

Updates #15269

Change-Id: If7fcaca7676e4a51f588528f5ced28220d9639a2
Reviewed-on: https://go-review.googlesource.com/22097Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>