Add unicast receive capability. Add MacOS support. Make both Linux & MacOS work.

[sbytnar]

Works for me. :-) Verified "discover" and "query" on MacOS & Raspbian. Made some notes in Known issues about inconsistent behavior. Did not verify "set". Some changes to README to describe the project better. Q: Is socket.SO_REUSEPORT really necessary? A: I enabled it in earlier passes to get earlier MacOS changes working, and left it in when I found it worked on Linux & MacOS. Doesn't hurt? The unicast changes may address: https://github.com/tabacha/ProSafeLinux/issues/33#issuecomment-711144598

[Z3po]

Thanks @sbytnar for providing this MR. I've merged it to master at https://github.com/Z3po/ProSafeLinux . I've been working on that Probject together with tabacha a long time ago and are now trying to reanimate the whole thing.

Feel free to check there for updates which (hopefully) come available soon :)

@tabacha if you'd like you could point to my repository as you do not want to maintain this anymore. I'll be happy to take over.