Convert mp_find_prime to use mp_prime_is_prime

Modify the signature of mp_find_prime() to permit the number of
Miller-Rabin rounds to be specified.  In addition, valid responses
now include MP_NO, MP_YES, and MP_VAL which is returned when
mp_prime_is_prime() fails.

Change-Id: I0195129a4dd75875e6dddb6d49a5ceb30afb1a17
libtommath: allow mp_shrink to shrink initialized, but empty MPI's

from https://github.com/libtom/libtommath

Change-Id: I5814caeb44db710957152a7d47b03f9beb6e5147
libtommath: mp_error_to_string: return const char* instead of char*

From https://github.com/libtom/libtommath

Change-Id: Idd5cbab380f5d504fece93afb3a5fcc026309482
libtommath: Fix mp_mod(a, b, c) if b < 0 and a = n * b, n integer

it used to return b, now it return 0.

From https://github.com/libtom/libtommath

Change-Id: I99f5d42c09a6fbddfb32b7dd0ed10ef04f1c08a5