(#22431) Re-enable windows executable external facts

This reverts the change made in fe5e639fd8, as starting with facter
1.7.3, external executable facts are supported on Windows.

Also added words to ensure the facts.d should only be writable by
Administrators, otherwise, a non-Admin could escalate privileges.
Merge pull request #208 from joshcooper/support_windows_external_executable_facts

(#22431) Re-enable windows executable external facts