factor: speed up Pollard-rho loop counters

factor: speed up Pollard-rho loop counters

* src/factor.c (factor_using_pollard_rho)
(factor_using_pollard_rho2, mp_factor_using_pollard_rho):
Use int, not unsigned long int, for counters that won’t go above
2**31 on practical platforms.  This yields a significant speedup
on GCC 15 x86-64, and using signed values allows for automatic
checks for overflow when using gcc -fsanitize=undefined.