clean up software rate limiter a little
clean up software rate limiter a little

also fixes the calculation of the frame length on the wire
(used to be off by a factor of the CPU frequency in GHz, but not really
noticable)

scholzd/MoonGen,gallenmu/MoonGen,scholzd/MoonGen,gallenmu/MoonGen,scholzd/MoonGen,gallenmu/MoonGen,gallenmu/MoonGen,dschoeffm/MoonGen,emmericp/MoonGen,gallenmu/MoonGen,gallenmu/MoonGen,gallenmu/MoonGen,emmericp/MoonGen,dschoeffm/MoonGen,dschoeffm/MoonGen,emmericp/MoonGen,gallenmu/MoonGen