add a sink node to a hierarchy
Set sink such that it may be transitioned into from any
other role in p but may not transition out. It may not
already be the parent or child of any other node as set with
sqlbox_role_hier_child(3) or a start node as set with
sqlbox_role_hier_start(3). There may be multiple sink nodes.
Setting multiple times as a sink is a no-op.
sqlbox_role_hier *p, size_t sink);
A sink is considered a sibling transition: it will not inherit any
permissions from other nodes.
Returns zero if sink is not a valid role, is a start role,
has already been assigned as a child or parent, or memory allocation fails.
Returns non-zero otherwise.