system/process fixes on AIX

[djboris9]

To be labelled:

• Bug
• Enhancement

What does this PR do?

It makes system/process usable on AIX. The preparation of this package for AIX was already done but in it's current state it's not usable for metricbeat.

The following things were done:

• GetPIDState was implemented for AIX as currently it was returning a "not implemented" from psutils
• Function procExists was implemented as the usage of GetInfoForPid isn't suitable to detect if a process still exists and can lead to errors. For example metricbeat couldn't handle lock files on AIX. Usage of getprocs on AIX possibly wrong #60
• A bug was fixed where in special cases (e.g. pid 0) no exe is parsable from cmdargs which caused out of range errors
• system/process tests were activated on AIX

Why is it important?

metricbeat is mostly prepared for AIX but it isn't completely working because of bugs like this or lacking features like #58
This PR ensures that a current metricbeat version is still buildable and usable on AIX.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

Author's Checklist

Related issues

• Closes Usage of getprocs on AIX possibly wrong #60
• Relates feat: Add AIX support for system/filesystem #58

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• Start Time: 2022-11-08T06:41:55.763+0000

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