[PATCH 0/2] config includes 3: revenge of the killer includes

So here's a third version of the config include patches that I think are
sane. For those of you who missed episode 2, our heroes decided to drop
the include-from-ref bits from the series, but otherwise incorporated
all suggestions. There was a surprise cliffhanger ending in which it was
discovered that suppressing duplicate include files was actually the
villain!

This version keeps a simple depth counter to stop cycles from causing
infinite loops, but otherwise allows multiple inclusion of files.

And now, the exciting conclusion...

  [1/2]: docs: add a basic description of the config API
  [2/2]: config: add include directive

-Peff