Port to MacOS

[jyn514]

This has been an ongoing saga, many thanks to Justin Baum for testing. I think the easiest way to do this is to get #22 going so I can test on my own.

Known subissues:

• [x] libmagic is not installed by default - ~documented in https://github.com/jyn514/crappy-server/commit/1bc4cce0a9b11f9b57e3e9f38e7aa25b08bdf074~ removed libmagic altogether in eda870253d16dadcf6a7e04f8a5ee9218b359ebe
• [x] system structs have fields with different lengths on Ma - closed in 459f681
• [x] system structs have different fields on Mac - should be closed in https://github.com/jyn514/crappy-server/commit/0a53c39da3e9ffa2af24e81d3d1dc09a5c6a7fcf but let's keep an eye on it