fix: package upgrades no longer ask the configuration questions.

debian-package/debian/config

@@ -2,6 +2,12 @@
 set -e
 . /usr/share/debconf/confmodule
 
+# An upgrade (configure with the old version) asks nothing; vpn-router.json holds the
+# configuration, and dpkg-reconfigure asks the questions again.
+if [ "$1" = "configure" ] && [ -n "$2" ]; then
+    exit 0
+fi
+
 # Every question has a default, so a non-interactive install with nothing
 # preseeded completes without prompting. The defaults are platform "none" and
 # mode "manual", which together mean: install the files, configure nothing.