type characteristics.
internal_types.h will contain only settings invisible from standard C, e.g.,
in the __* or _[A-Z]* namespace, and be reused by files like limits.h.
This allows us to shorten machine/limits.h greatly, as all the common defines
are now in sys/limits.h, plus a small stub in internal_types.h.
Tested on all arches as far as I know.
Approved after discussion with art, millert, deraadt, and others.