Slightly refactor 'Additional Network Interfaces' page.

[jheretic]

This is a refactoring of the 'Additional Network Interfaces' page in the basic menu. It allows a user to control the gateway ethernet port's DHCP settings, as well as optionally provide a static IP configuration. It also controls whether a present gateway is advertised to the rest of the mesh, and exposes the ability to mesh over ethernet in two circumstances:

• When we are attempting to receive an IP address via DHCP.
• When we have statically assigned IP information.

We don't provide it in circumstances where there is or may be an HNA client network on the ethernet interface, because OLSR does not permit the same interface to be both mesh and an HNA. There are edge cases that aren't covered, but trying to support them would both take significant revisions and result in potentially unexpected behavior.

To test:

• Go to the additional network interfaces page, and test that all of the DHCP options work.
• In the two cases where mesh-over-ethernet is supported, test to make sure you can mesh with another node over the ethernet interface.

Partially addresses https://github.com/opentechinstitute/luci-commotion/issues/209, and makes the process mentioned in https://github.com/opentechinstitute/luci-commotion/issues/64 largely unnecessary.

[dismantl]

Does this allow clients from different mesh segments (i.e. wireless versus wired) to communicate to each other? I thought firewall rule changes were needed for that.

[jheretic]

That is a different issue. This is just expanding the capabilities of the interface to additional use cases.