Remove L4 support from main tree.

iprt,VBoxServiceAutoMount.cpp: RTPathSetOwner[Ex] should take NIL_RTUID/GID not -1 / ~0, the only excuse is that NIL_RT[UG]ID probably didn't exist when it was written. Ditto for the RTFileSetOwner API (not realized yet).

iprt: try split out some of the RTPathQueryInfo* dependent bits into separate files in the posix world.