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.

bin/tests/system/ifconfig.sh.in

@@ -24,6 +24,10 @@
 # We also set the MTU on the 1500 bytes to match the default MTU on physical
 # interfaces, so we can properly test the cases with packets bigger than
 # interface MTU.
+#
+# See also org.isc.bind.system (a version of this script for use on macOS)
+# and testsock.pl (which checks the interfaces are configured)
+#
 
 top_srcdir=@abs_top_srcdir@
 
@@ -237,6 +241,10 @@ sequence() (
 # 'ns' in the namserver being configured.
 # 'int' interface identifier.
 #
+# See also `org.isc.bind.system`.
+#
+# This `max` setting is grepped out for use by testsock.pl
+#
 max=11
 case $1 in
     start|up|stop|down)