The system tests are using another IP address

The system tests are using another IP address

Reduce the number of places that know about the number of IP addresses
required by the system tests, by changing `testsock.pl` to read the
`max` from `ifconfig.sh.in`. This should make the test runner fail
early with a clear message when the interfaces have been set up by an
obsolete script.

Add comments to cross-reference `ifconfig.sh.in`, `testsock.pl`, and
`org.isc.bind.system` to make it easier to remember what needs
updating when an IP address is added.

(cherry picked from commit 258a896a0cf9b26be64f80c3b9fba7b0d5d43f50)