Redirect stdin

Invoke powershell so it reads the script to execute from redirected stdin. This eliminates possible security issues resulting from powershell script being displayed on the command line or in log files, as well as the 32K character maximum length for command lines. It also resolves 'number3' in #1.

Also, we no longer create top-level constants, e.g. `::POWERSHELL`. Previously, ruby would issue warnings that the constant was redefined during pluginsync #2. This is now resolved.