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

Change-Id: I7750ae3cb1e923a13e0035d58d58a81c35e7e86b
Reviewed-on: https://go-review.googlesource.com/128775
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>