Simplify BN_nnmod().

Simplify BN_nnmod().

In the case that the result is negative (i.e. one of a or m is negative),
the positive result can be achieved via a single BN_usub(). This simplifies
BN_nnmod() and avoids indirection via BN_add()/BN_sub(), which do BN_cmp()
and then call into BN_uadd()/BN_usub().

ok tb@