A checkpoint of a part of a work-in-progress. Some more cleanups for

A checkpoint of a part of a work-in-progress.  Some more cleanups for
config(8).  This commit allows control of the creation of the
#include "foo.h" files.  We now only create them explicitly when needed.
BTW; these are mostly bad because they usually imply static limits on
numbers of units for devices.  eg: struct mysoftc sc[NFOO];
These static limits have Got To Go.