When using jtreg on Windows, it would be nice if it was not necessary to use the -wsl option, by improving the default behavior.

Given we already have the -wsl option, the proposal is to add a similar new option -cygwin.

Both options should only be recognized on Windows.

-wsl forces WSL mode; -cygwin forces Cygwin mode; if neither are given, the mode is determined by looking at the environment. When running under WSL, WSLENV is set; when running under Cygwin, "cygwin" is in the PATH. In the unlikely event of ambiguity (cygwin on path and WSLENV set), use one option or another to override.