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 258a896a)

Closes #3519 (closed)