div2 and nails

[Kevin Ryde]

Torbjorn Granlund <tg at swox.com> writes:
>
> We intensionally don't define how all internal interfaces
> work when GMP_NAIL_BITS != 0.

longlong.h is unchanged (ie. full limb values).

Most macros in gmp-impl.h expect nails.  udiv_qrnnd_preinv is a
notable exception, it takes full limbs at the moment, but we're not
decided on what it and invert_limb should do.  For nails the
thresholds (eg. DIVREM_1_NORM_THRESHOLD) are setup to mean preinv is
not used.