add a start (source) node to a hierarchy
Set start such that it may transition into any other role
in p but that no other role may transition into it.
These are usually called “sources” but called starting nodes to
disambiguate from sqlbox_role_hier_src(3). The node may not
already be the parent or child of any other node as set with
sqlbox_role_hier_child(3) nor a sink node. There may be
multiple starting nodes. Setting multiple times as a start is a no-op.
sqlbox_role_hier *p, size_t start);
A start has sibling transition to all other nodes: it will not
inherit any permissions from other nodes.
Returns zero if start is not a valid role, has already
been assigned as a child or parent, is a sink, or memory allocation fails.
Returns non-zero otherwise.