sys/tree.h: Red child with black sibling rotations

sys/tree.h: Red child with black sibling rotations

Add specialized rotations RB_RED_ROTATE_LEFT() and RB_RED_ROTATE_RIGHT() which
may be used if we rotate a red child which has a black sibling.  Such a red
node must have at least two child nodes so that the following red-black tree
invariant is fulfilled:

  Every path from a given node to any of its descendant NULL nodes goes through
  the same number of black nodes.

      PARENT
     /      \
  BLACK     RED
           /   \
        BLACK BLACK