Migrate to using golang.org/x/sys/unix over syscall

[bored-engineer]

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• I've read and agree to the Code of Conduct.
• I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
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• I've written tests to cover the new code and functionality included in this PR.
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PR Summary

The "syscall" package used by go-audit 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.

This PR migrates to using golang.org/x/sys/unix instead as new fixes/features are implemented there.

Related Issues

Closes #73

Test strategy

go build succeeds without errors
go test passes

[CLAassistant]

All committers have signed the CLA.

[bored-engineer]

Sorry for the force pushes, had to fix the emails in the commit to match the CLA email I used

[codecov-io]

Codecov Report

Merging #74 into master will not change coverage.
The diff coverage is 84.21%.

@@           Coverage Diff           @@
##           master      #74   +/-   ##
=======================================
  Coverage   64.09%   64.09%           
=======================================
  Files           7        7           
  Lines         596      596           
=======================================
  Hits          382      382           
  Misses        193      193           
  Partials       21       21           

Impacted Files	Coverage Δ
client.go	89.23% <80.00%> (ø)
audit.go	74.58% <100.00%> (ø)
marshaller.go	52.43% <100.00%> (ø)
parser.go	89.61% <100.00%> (ø)

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[iFrozenPhoenix]

@wadey is there the possibility that this pull request can be merged?

[psanford]

I'm unclear as to what problem this is solving. While it is true that syscall is a frozen API, it has the necessary netlink APIs to get kernel audit data. The syscall package still gets bug fixes, it is just not getting new API additions.

[iFrozenPhoenix]

Thank you for the clarification why it is not merged.