feat: Android compatibility

[dignifiedquire]

Unfortunately the default ways are not as easily used on different Android versions.

This implements two different versions for doing this on Android, which should allow supporting Android back to NDK API version 16 (the one that I need currently) up to today.

The first approach is to use dlopen to load libc and use getifaddrs which should work on API 24 and up. The headers are unfortunately not exposed, which is why this workaround is needed.
For all other versions where the symbols are not available, a backup using netlink is implemented.

This is currently still being tested, which is why I am opening this as a draft, for feedback if this would be merged once it is confirmed to be working.

(Sorry for the whitespace changes, I ran cargo fmt on the files I was working on)

[link2xt]

Is this ready for review?

(Sorry for the whitespace changes, I ran cargo fmt on the files I was working on)

I opened #23, would be nice if the project source was formatted already.

[dignifiedquire]

Is this ready for review?

Yes, tested and works now in the described cases.

[link2xt]

@dignifiedquire Looks like dlopen is not maintained: szymonwieloch/rust-dlopen#47
There is a maintained dlopen2 with updated dependencies.

[flub]

@shellrow we had some discussion about whether it's worth switching to dlopen2 or not and were rather inconclusive. So it's up to you really, would you prefer us to switch over or leave it as is?

[shellrow]

@flub Sorry for the late reply. We should migrate to dlopen2 which is still maintained.
(I'll continue to maintain default-net, although I don't have enough time at the moment)

[flub]

@flub Sorry for the late reply. We should migrate to dlopen2 which is still maintained.

Cool, done.

(I'll continue to maintain default-net, although I don't have enough time at the moment)

Thanks! If you like we [0] would always be happy to help out maintaining as we'll be relying on this and keep having stakes on it working on lots of platforms. Of course I know that's a tall demand from people you've never known before... so don't worry if you'd rather decline. But also don't hesitate to ask whatever you'd like to see done. We can at least make it as simple as hitting the merge button on a bunch of PRs.

[0] That would be the n0 and deltachat teams: https://github.com/n0-computer and https://github.com/deltachat

[shellrow]

Thanks!