Migrate from "syscall" to "golang.org/x/sys/unix"

[bored-engineer]

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

This is more of a best practice as I'm not aware of any specific issues caused by the older dep.

As of today go-audit uses the "syscall" package which has been frozen since Go 1.4:

Deprecated: this package is locked down. Callers should use the corresponding package in the golang.org/x/sys repository instead. That is also where updates required by new systems or versions should be applied. See https://golang.org/s/go1.4-syscall for more information.

go-audit should migrate to using golang.org/x/sys/unix instead as new fixes/features are implemented there.

This should be a pretty straightforward migration as most structures are the same in both packages, any concerns with me sending over a pull request for this?