[Lazarus] Jumping between files in debugger

[Juha Manninen]

I simplifiled the code dealing with debugged file names in r55545.
Originally a bug related to symlinks was fixed:
  https://bugs.freepascal.org/view.php?id=31577
but the fix was more complex than necessary.
Now it is simpler. Please test everybody. Everything related to
debugging should work as before.

Martin, what do you say about the changes?
I wanted to write this here to get more testers.

I also noticed code that is never called on Linux. I added an
exception for testing but reverted it soon because the code was called
on Windows.
Why only Windows, I have no idea.

Juha