Since f-stack run with polling mode, nginx will call gettimeofday every loop, and cost a lot.
With this commit, f-stack will update current timespec periodically in
ff_hardclock_job. And ff_gettimeofday will get this value.
In nginx, hijack gettimeofday to call ff_gettimeofday.
RX: VLAN strip, CRC strip, IP/TCP/UDP checksum.
TX: IP/TCP/UDP checksum, TSO.
Note that TSO has a problem: when enable TSO,
F-Stack is much slower(Temporarily not resolved).
So TSO is disabled by default.
Edit config.ini to enable it.