Add the current revision of the debug symbols patch.
Some fixes:
- Update usr mtree file.
- Add separate rules for ports.
Use a variable for the "full" binary or shared library.

Fix breakage of the beforelinking target.
defined(MK_DEBUG_FLAGS) --> ${MK_DEBUG_FLAGS} == "yes".
Fix checking of MK_DEBUG_FILES.
Make sure that beforelinking explicitly depends on the object files.