Create CLI option to bind to UNIX socket

[Xowap]

As suggested in #264, this PR brings the ability for tusd to bind to a UNIX socket.

The implementation adds a -unix-sock CLI option which if set to a non-empty value will bind tusd to the provided path and discard the TCP host and port.

I'm unsure about the standards of the project, so please me how I can improve this.

[Acconut]

Thank you for this PR, it looks very good, all in all. I will try it on my own in the next few days and come back if any problems occur. In the meantime, do you know how this will behave on Windows (or other platforms not supporting UNIX sockets)?

[Xowap]

Good!

Regarding the Windows support, that's an excellent question, I suppose that an error will be raised by the go standard library. I've found this issue which suggests it's going to fail yet it's going to be implemented one day. As it's specific to the implementation of Go, I'd suggest to keep the option available in Windows so that if the code is compiled on a compatible implementation then it automatically works. Though it might get confusing, maybe do that and document it in the help message?

In any case, I'll have to test it.

[Acconut]

Apologies for my delay. I didn't know that Go on Windows recently gained support for UNIX sockets. In that case, we should just let Go take care of testing support.

[Xowap]

Awesome 👍

[Acconut]

I am currently working on a few smaller improvement to tusd, so the UNIX socket support will be shipped in the new release in a few days. And thank you again for the contribution