Required a little soft-coding in areas that could be thought of as
"pluggable" firewall rules.  It looks very doable for 17.7, we could
potentially make IPsec and OpenVPN plugins!  Or at least prove that
the plugin framework is capable of plugging in a complex VPN tech.

This looks simple enough now.  Discussed with @adschellevis.

Spotted by: @adschellevis

The array was still there, and even though empty evaluated as
being filled with data that requires an interface.

With the previous call-sites in place we don't need this
"fixup" anymore, assuming that interface groups, OpenVPN
and IPsec settings are now consistent under the new state,
which is true, because right now it was enforced by the
rc system anyway.

Phase 1 add/delete is the best call spot for this as it
defines a "physical" connection.

This wasn't completely right before, trying to plug into rules
reload paths and caused issues with people adding/deleting their
OpenVPN devices, but the rules tab not properly indicating this
since OpenVPN has become a pluggable interface.

(auth, totp) isolate TOTP functionality into a trait, refactor LocalTOTP. all needed for https://github.com/opnsense/core/issues/1030

(model, fieldtypes) add update only text field type, closes https://github.com/opnsense/core/issues/1045

(status_gateway_groups) show missing gateways and status "monitoring disabled", for https://github.com/opnsense/core/issues/1132

(openvpn) add "Use common name" option to control username-as-common-name, closes https://github.com/opnsense/core/issues/1154

Suggested by: @fabianfrz

Suggested by: @Antonio-Prado

Now always restore from a full backup only, the standard has already
been switched in the scope of this ticket a while back.  The section
restore could be improved, but it's not really worth doing this in
the old code anymore.  Hopefully the last change of its kind here.

(legacy, config) add legacy glue for toArrayFromFile() into load_config_from_file, closes https://github.com/opnsense/core/issues/1097

(model) refactor getConstraintValidators() in BaseField, add getConstraintByName() to retrieve a constraint by name/key and add functionality to explain the model field relations.

Using reference tags in the constraints section its possible to tell the model which fields interact to avoid the need for a full model validation after each change.
For example if fieldA has a constraint which relates to fieldB, the latter can add a reference to trigger validation when only fieldB changes.

<Constraints>
   <check001>
     <reference>fieldB.check000</reference>
   </check001>
</Constraints>