Revert a sysrc to an echo as it due to invalid characters.

When running

sysrc -f /etc/sysctl.conf net.inet.ip.fw.verbose_limit=5

it throws the following error:

sysrc: net.inet.ip.fw.verbose_limit: name contains characters not allowed in

shell

Fix this by reverting to using an echo to add the sysctl to sysctl.conf.

Reported by: Laurent Gilbert (lg.athome@free.fr)
Event: Aberdeen hackathon 2019