factor: prefer signed types

factor: prefer signed types

When it’s easy, prefer signed types to unsigned, as
they are less confusing and allow overflow checking.
* src/factor.c (struct mp_factors, udiv_qrnnd)
(count_leading_zeros, count_trailing_zeros)
(factor_insert_multiplicity, mp_factor_clear, mp_factor_insert)
(factor_insert_refind, factor_using_division)
(mp_factor_using_division, powm2, millerrabin, millerrabin2)
(mp_millerrabin, prime_p, prime2_p, mp_prime_p, isqrt, isqrt2)
(invtab, q_freq, factor_using_squfof, strto2uintmax)
(print_factors_single, main):
Prefer signed integers to unsigned.