Nifty Perforce - belkiss' fork - 2.0.9

• Support for vs2017 (min 15.8) and vs2019
• Fix detection of environment variable P4DIFF
• Fix process status in case an exception is thrown. Before that change, if a process started by nifty failed, it would still be considered running by nifty.
• Added partial support for loading the Nifty Perforce extension in the background (from @jgvincen)