I've moved it from mpunsafe.c into the main mpint.c, and renamed it
mp_mod_known_integer, because now it manages to avoid leaking
information about the mp_int you give it.
It can still potentially leak information about the small _modulus_
integer - hence the word 'known' in the new function name. This won't
be a problem in any existing use of the function, because it's used
during prime generation to check divisibility by all the small primes,
and optionally also check for residue 1 mod the RSA public exponent.
But all those values are well known and not secret.
This removes one source of side-channel leakage from prime generation.
