Port mksysnum_{(open|net|free)bsd,dragonfly,darwin}.pl Perl
scripts to mksysnum.go.

mkall.sh script is modified to run mksysnum.go. Running
./mkall.sh generates no git diff besides command name and
function prototype in comments of generated files.

Updates golang/go#27779

Change-Id: I534734a9c7fda186ca133ba19e14481e55832be1
Reviewed-on: https://go-review.googlesource.com/c/152677Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>