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

Change-Id: I00d38d0e528900be7c5afca05c8dc50ee7628b20
Reviewed-on: https://go-review.googlesource.com/129317
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>