The duplication of _Kind and kind constants is a legacy of the
conversion from C.

Change-Id: I368b35a41f215cf91ac4b09dac59699edb414a0e
Reviewed-on: https://go-review.googlesource.com/15800Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>