create new machine/_float.h which is namespace clean. create a new
MI float.h which pulls in and defines the values that are needed from
there, and repair sys/limits.h so that it defines the values it needs
as well (depending on POSIX version, XPG version, etc). guenther has
a more exact selection of that coming for limits.h.
this also fixes a few mistakes for the vax.
reviewed by kettenis and guenther.