(PUP-2732) Use less restrictive exec user validation

See https://tickets.puppetlabs.com/browse/PUP-2732 for more information.
The `Etc` module is available, but those methods appear to return nil:

<pre>
C:\work\puppet>irb
irb(main):001:0> require 'etc'
=> false
irb(main):002:0> Etc.getpwuid(0)
=> nil
</pre>