systemd-sysctl applies /etc/sysctl.conf AFTER all of sysctl.d/, so the
stale ip_unprivileged_port_start=80 line there beat every drop-in and kept
the runtime value at 80 after the first deploy pass. deploy_pihole now
comments out an active line in /etc/sysctl.conf (idempotent) and generalizes
the sysctl.d conflict scan to any file setting a value above 53.