Eliminate every second multiple

This is a _very_ simple optimisation done to src/sieve.c which keeps adding
two instead of one to the factor f. It boosts the sieve nevertheless
noticeable. I updated the benchmarks accordingly.

There was also the time for a 10e11 pt added which was generated in nearly
15 minutes. According to the estimated complexity I should finally be able
to generate my 10e12 pt in about 3 hours using 120GiB of memory if I only
had those...