Make bugs only pointable with Bug Net

[cx384]

Problem

Sometimes a firefly or a butterfly is in your way when you want to dig something, and it can't be destroyed by punching it. It basically behaves like punching stone with a bare hand. This is unreasonable and annoying.

What does this PR do?

This PR makes fireflies and butterflies only pointable using a bug net. It utilizes the new pointabilities feature.

Alternatives

• Make fireflies and butterflies digable without dropping anything.
• Move fireflies and butterflies to an adjacent position if someone tries to punch them.

[cx384]

Yeah, the compatibility is only for old Minetest versions which use a new MTG version. I don't know how compatibility issues are typically handled in Minetest, but the first two solutions you suggested are too much imo, considering how insignificant the problem is. I'm not even sure if the legacy support I added is needed, since a new MTG version may depend on Minetest 5.9.0 anyway.

The only good way I see to solve this is to add a feature to Minetest which allows sending different item definitions to clients depending on the client version. However, this would be a huge effort and it will rarely be used. So I think it is reasonable to just ignore it, in case this PR gets merged.

[sfan5]

To be clear (in the past) MTG did require the server to be up-to-date but we still couldn't use something (e.g. formspecs) that flat-out did not work on older clients.