pkg can display to-be-deleted packages, but won't resolve these
issues when triggered using "upgrade -n".  Thus, it's pointless
to try to keep this code until a newer pkg release can actually
cope with resolving the full upgrade cycle before upgrading.

This is the glue needed to be entirely plugin-agnostic, it only
needs new hooks for spots we haven't pluginified yet.  The dns
subsystem is another likely candidate in the long run.

Allows to move these plugins away from core: bsnmpd, igmpproxy,
relayd, miniupnpd.  ACL and menu entries pending...

.. with either IGMP or FTP proxy.

It's difficult to say we can remove Relayd from base just yet.
Unbound and Dnsmasq sure are not going anywhere due to their
dependencies into DHCP.  Relayd is similarly intermingled.

fix bug in firewall_rules_edit.php reported by tillsense in the forum…

fix bug in firewall_rules_edit.php reported by tillsense in the forum at https://forum.opnsense.org/index.php?topic=3951.0;topicseen

The amount of work required in the past two years for this
original pfSense 2.2 item has been immense.  Still deleteing
unused functionality like the cache, "reload" not being
implemented, DHCPv6 static registration missing in action.

While there, dedup the dnsmasq static DHCP registration.

PR: https://github.com/opnsense/core/issues/1250
PR: https://github.com/opnsense/core/issues/624