firewall, plugin. move filter_generate_ipsec_rules() out of the way, move to plugin type rules later.

firewall, plugin. start moving user generated rules into the same logic as the plugins. step by step.

First enable our gateways and gatewaygroups on our plugin system and setup reply-to attributes in interface metadata (could be temporary, but good enough for now)

firewall / plugin, add getInterfaceGateways() and additional metadata to fetch all related gateways. Needed for some of the current internal rules.

OPNsense opnsense: /interfaces_vlan_edit.php: The command `/sbin/ifconfig 'em2_vlan1' vlandev 'em2' vlan '1' vlanpcp '0'' failed to execute
OPNsense opnsense: /interfaces_vlan_edit.php: The command '/usr/sbin/ngctl name 'vlan1': 'em2_vlan2'' returned exit code '71', the output was 'ngctl: send msg: No such file or directory'

interface_bring_down() doesn't use a real interface name as call argument, mwexecf('/usr/sbin/ngctl name %s: %s', array($tmpvlanif, $vlanif)) isn't used by my knowledge.

firewall_rules, keep category selection when changing tabs. closes https://github.com/opnsense/core/issues/1592

guiconfig.inc, add class to top_tab so we can find them easier, for https://github.com/opnsense/core/issues/1592

* Plug gateway fixup "feature" into filter_configure_sync(), the
  only point where it should matter as everything else is GUI
  trigger-happy disruption.

* Discourage the use of the "feature" by moving it into the firewall
  advanced settings, adding a deprecated note.

* Use the new system_default_route() call to avoid code drift.