Also generate the constants using mkerror.sh instead of having to list
each of them manually in types_dragonfly.go

Change-Id: I89b933834be6aa0eeec3d5ddbbf610e693b29327
Reviewed-on: https://go-review.googlesource.com/129316
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>