Swap in single_test.py as the new scansys.py.

It's not feature-complete yet, but it is getting there, and there's no
point pretending that the old scansys.py is still useful.  (Haven't yet
actually deleted it, though.)
Add some obsolete headers mentioned in the Autoconf manual.

They are so very obsolete that it might be a good idea to take
them *out* of the Autoconf manual.  Answering questions like that
is exactly what this project is about.
Nuke the conform/thread booleans being passed around everywhere.

There is now a CompilationMode object, which is currently just
a wrapper around those same two booleans, but will become more
capable in the future.  More importantly, the mode is now a
property of the *dataset*, rather than each Header object carrying
info for all N modes.  This will make output hairier (and may well
be the final nail in the coffin of the current output format) but
it's worth it for how much tidier the code performing the actual
tests is.