Adapt userlandfs kernel-addon to atomic_* API changes.

* Remove volatile qualifiers, use atomic_get() for safe access

* Fixes build of userlandfs
Adapt kernelland_emu to kernel API changes

* Adapt to changes in kernel synchronization APIs.

* Makes userlandfs work again.
Use BPathFinder to find userlandfs add-ons

* ...to make it work with packaged and non-packaged.
Debugger: Fix minor issue noted in #11003.

- Reset tooltip when it's no longer needed (i.e. if the file path changes).
  Otherwise, we'd erroneously show the old tooltip if the new path was short
  enough to no longer be truncated, as would often happen after locating a
  missing source file.
Build fix.