Palisade: replace dfs_dsf() with a simple iteration.

The whole purpose of a dsf is that you can traverse the edges of your
graph in any order you feel like. So if you want to build the
connected components of a graph you can just loop over all the edges
once. There's no need to run a depth-first search.

In fact there were an amazing number of things wrong with this 10-line
function:

 - As Ben points out in commit 21193eaf9308ace, it didn't bother with
   bounds checking when searching the grid, instead relying on the
   never-removed grid boundary to stop the search - which was fragile in
   the face of other bugs.

 - The recursion uses linear stack, which is much worse than linear
   heap, since stacks are often much more limited. (And the dsf _also_
   used linear heap.)

 - The recursion was completely unnecessary.

 - The function used internal knowledge about dsf.c in order to define
   the value UNVISITED to match what would happen to work.

 - The name 'dfs_dsf' is totally confusing and almost impossible to
   type!

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