Count probe routes

A network team counts the diagnostic routes a probe could take. link_ms is a square table where link_ms[a][b] is the one way latency in milliseconds of the link from node a to node b, and 0 means there is no link, including every entry on the diagonal. The table is not always symmetric. A route starts at node 0, ends at the last node, never visits a node twice, totals at most budget milliseconds, and passes through every node listed in must_visit. must_visit may repeat a node and may name the start or the end. Return how many such routes exist. Return 0 when the table has no nodes. A table with a single node gives one route, the one that has already arrived, whenever budget is not negative. There are at most 12 nodes.