- add variable for the null device filename and use that in Perl code.
- initialize this variable with `NUL` on native Windows.
- add `%DEV_NULL` variable and use it in tests.
Fixes `The system cannot find the path specified.` messages seen when
running `runtests.pl` with native Windows Perl.
Also adjust code to not break mcedit syntax highlighting.
Cherry-picked from #14949Closes#15098
