Should now work properly on Windows, DOS, etc., too

Run::Parts::Perl specifically accepts suffixes if running on Windows.

Also adds a specific test to test this new feature -- which runs only
on DOS-ish OS.
Workaround exec issues on DOS-ish platforms

Apparently backick-based execution does not work with forward slashes.

Also adds a test for the "run" command on DOS-ish platforms, just
based on batch files (.bat) as I don't want to include .exe or .com
files.
Drop unneeded variable from t/win-functionality.t
Code-Deduplication: Add function dosish for the dos-ish test
RPP: Move variable initialisation behind the POD intro
RPP: Add POD section about file name constraints
Mention where the file name constraints are documented