Provide bn_div_rem_words() and make use of it.

Provide bn_div_rem_words() and make use of it.

Provide a function that divides a double word (h:l) by d, returning the
quotient q and the remainder r, such that q * d + r is equal to the
numerator. Call this from the three places that currently implement this
themselves.

This is implemented with some slight indirection, which allows for per
architecture implementations, replacing the define/macro tangle, which
messes with variables that are not passed to it.

Also remove a duplicate of bn_div_words() for the BN_ULLONG && BN_DIV2W
case - this is already handled.

ok tb@