Capsicum support was added by https://golang.org/cl/47290, but only for
amd64. This broke the freebsd/386 build which was subsequently fixed by
http://golang.org/cl/51270. However, even with this fix the tests still
fail to build due to the missing Capsicum functions and types.

Fix the build/test on freebsd/386 for real by re-running mkall.sh on
FreeBSD 11.1-RELEASE (inside VirtualBox) to get all the Capsicum
functions and types.

Some zerrors constants were removed or converted to enums in the FreeBSD
headers. Manually move them to a dedicated file. Do the same for amd64,
where these constants were removed by accident in
https://golang.org/cl/47290.

Updates golang/go#21136

Change-Id: I54fe2c4567674b07dedad677756369a43a1f0ef6
Reviewed-on: https://go-review.googlesource.com/51570Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>