Support windows internal commands

[mvz]

Summary

Support windows internal commands by accepting them as existing commands.

Details

Allow SpawnProcess to find built-in shell commands in addition to real executable files. Provide list of such built-in commands for Windows.

Motivation and Context

• This was implemented as part of Improve AppVeyor Build #505
• Fixes [Windows] Aruba doesn't know how to run "built-in" DOS commands #568

How Has This Been Tested?

TBA

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (cleanup of codebase without changing any existing functionality)
• Update documentation

Checklist:

• I've added tests for my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[mvz]

@xtrasimplicity I extracted some of your work on #505 to this pull request. For some reason, specs are all green on Appveyor without these changes, which tells me that it somehow exposes some unix utilities. Any idea how to disable that?