rc.conf: Update commented examples for lo0 to use CIDR

[jhb]

Summary

In particular, a bare IP address no longer works.

[bz]

libexec/rc/rc.conf

266–267

Technically this should be sysctl -n net.inet.ip.loopback_prefixlen; adding @emaste for this given he looked into enabling some of the experimental draft bits lately.

268

We could also change those to /64 and be done but that's for another reason.

[emaste]

libexec/rc/rc.conf

266–267

Maybe add # See the net.inet.ip.loopback_prefixlen sysctl? Or we just leave this alone until/unless we change the default and update this commented setting at the same time.