Debugger: Improve gdb binary settings handling.

- Remove non-executable entries  on reading (check only
  absolute paths since path search is too expensive).
- Make sure gdb binary settings are not written out unless the
  user really changes them.

Reviewed-by: aep <arvid.picciani@nokia.com>
Acked-By: hjk
Reviewed-by: Tim Jenssen <tim.jenssen@nokia.com>