Only allow one choco.exe process to run at a time

This is meant to correct some possible issues that could and have occurred when running multiple processes at the same time.

Start by tracing the choco.exe process startup path and identify where concurrent invocations are handled. Review the existing issue discussion for the reported multiple-process scenarios; done means concurrent choco.exe runs are prevented without disrupting normal package-manager use.

Written by the indexing model from the issue text.