interfaces: improve the behaviour of "Use IPv4 connectivity"

[fichtner]

Important notices

Our forum is located at https://forum.opnsense.org , please consider joining discussions there in stead of using GitHub for these matters.

Before you ask a new question, we ask you kindly to acknowledge the following:

• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

The ironic truth about the option is that "Use IPv4 connectivtiy" opt-in is that the opt-in is actually the default so the default gets inverted, but only when PPPoE is involved in the first place.

It would be nice to restrict this to PPPoE and invert or migrate the current default in order to make the most sense and avoid a bit of code (and feature taint having the option always available causing undefined behaviour on non-PPPoE connections).