Add unix socket support

[clarfonthey]

Enables listening on unix sockets by specifying a file path for the bind address. Fixes #542.

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[thebaer]

Great addition -- thanks for submitting this! I'll test as soon as I can. In the meantime, anyone else should feel free to review!

[clarfonthey]

I did run the automated tests and everything passed, although I haven't gotten around to testing the actual server yet. Technically other options for unix sockets could be added, but I figured I'd choose the option with the minimum surface area for now since I'm not sure how much that's desired.

Will add some extra remarks as needed once I get around to that.

[clarfonthey]

Okay so, I finally got around to testing this, noticed a few issues, and fixed them. Notable changes:

1. Error handling in golang is awful and I accidentally shadowed err, which is now fixed
2. The server needs to delete any leftover socket files in order to actually listen on the socket; if it doesn't have permission
   to do this, it will just throw an error
3. You need to update the permissions of the socket after listening to ensure any other than the writefreely user can access the socket (while not strictly required, good practice)
4. I found out that http.Serve(listener, r) exists so I did that instead of (&http.Server{Handler: r}).Serve(listener)

I assume you still want to do some testing of your own, but at least as far as I'm concerned, this should be ready for review now.

[thebaer]

I went ahead and cleaned the code up to be a little more idiomatic, and made it so the socket file is deleted on server shutdown.

I probably undid your error-naming changes -- I'm not sure what the error shadowing issue was that you mentioned, since past code was overwritten by force-push. But let me know if this causes any issues! Otherwise I tested and it works well, so I'll merge this soon.

[clarfonthey]

I don't remember what the error change was, but I was particularly frustrated with the compiler in that moment and the brute force approach worked. So, if your code works, that's good.

I have a particular problem with force-pushing, so, I should get better about that.

I didn't remove the socket initially since I figured it didn't matter much since it'd be overwritten on the next time it starts, but it is an improvement. Thank you.

Everything looks good to me. Appreciate you tidying this up for me.

[thebaer]

Merging this now. Thanks again for working on this!