factor: 25% speed-up, on output

factor: 25% speed-up, on output

* src/factor.c (print_factors_single): Use fputs and umaxtostr
rather than printf with "%ju".  This reduced the time required
to compute and print the factors of the first 10^7 integers from
over 8 seconds to 5.75s.  Run this command:
seq $((10**7)) | env time factor > /dev/null