Make mp_unsafe_mod_integer not be unsafe.

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.

mpint.h

@@ -257,6 +257,12 @@ void mp_divmod_into(mp_int *n, mp_int *d, mp_int *q, mp_int *r);
 mp_int *mp_div(mp_int *n, mp_int *d);
 mp_int *mp_mod(mp_int *x, mp_int *modulus);
 
+/*
+ * Compute the residue of x mod m, where m is a small integer. x is
+ * kept secret, but m is not.
+ */
+uint32_t mp_mod_known_integer(mp_int *x, uint32_t m);
+
 /*
  * Integer nth root. mp_nthroot returns the largest integer x such
  * that x^n <= y, and if 'remainder' is non-NULL then it fills it with