(filter) cleanup code, aliasses, globals, etc. loosly related to https://github.com/opnsense/core/issues/1396

(filter) add prio, set-prio to new style FilterRule class as well (plugin support + single parsing in 17.7)

make the ipv4 subnet matching code work like the ipv6 code, fixes bug in subnet matching on tun interfaces

(ipsec) add mobike switch, change leftsendcert to always. for https://github.com/opnsense/core/issues/1393

* Add priority settings to advanced firewall rule options

* fix all the previous issues with the priority rules patch and add support for the 2 prio setting

* make the advanced options auto expand, even with various priorities set to 0

Align this some more: legacy_vlan_tag() is used nowhere else,
so it's ok to change the function and clarify the text usage.

* add VLAN Priority (PCP) setting to VLAN config

* use array() instead of [] here

* add a default value for $pcp to legacy_vlan_tag()