Normally, each RMIB subtree consists of an array of nodes, indexed
by node identifier. In a sparsely filled subtree, most of the array
is empty and just wasting memory. In that case, it may be beneficial
to have a level of indirection, with an intermediate array containing
pairs of node IDs and pointers to the actual nodes. This patch adds
support for such indirection.
For the use cases that inspired this patch, net.inet and net.inet6,
the indirection shaves off a little under 16KB of memory from the
TCP/IP service.
