Instead, get a list of known sysctls in the interesting prefix (net.inet.ip)
and check if there's an entry in the list for each sysctl we want to change.
If there isn't, then don't try to change it.

This fixes a problem with FreeBSD, which doesn't have
net.inet.ip.scopedroute but also doesn't need it.  Probably also fixes MacOS
10.5, which probably didn't have that either, but I don't know for sure.

Reported by Ed Maste.