Implement Dijkstra's algorithm for one-source shortest-path.

This uses the lpq module as the implementation of the priority queue.  That
means this implementation is some way behind the theoretical efficiency of
Dijkstra's algorithm.  It should be reasonably straightforward to swap out
the priority queue for a better one in the future, though.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>