Add the old <center> option for the placement policy. (Bug 5946)

Original commit messages:
. Reformat to move closer to house style
. Add center on top of leat overlap place algo
. Add sentinel value to edge arrays
. Use a Size instead of a Rect for a centering field
. Fix off by one bug
. Need to declare dx and dy
. Pass length of edge array instead of recomputing
. Fix missing open-brace in config.c
. Address the more trivial subset of danakj comments
. Revert "Remove now-unused config_place_center option."
  This reverts commit 5e282dae08be3b900e0337efa0fae8f3ffa92cd7.
. Remove reliance on sentinel value when scanning edge arrays
. Avoid need to initialize Size structure by removing it :)
. Clean up field expansion code somewhat
. Compress code further by using a structure for common args
. Fix search for next grid point
. Squeeze it even more by not using Size at all