call pcap_setnonblock() after pcap_activate()

libpcap looks expecting the function call order, that
pcap_setnonblock() is called after pcap_activate().

This makes TPACKET_V3, TPACKET_V2(USE_PCAP_IMMEDIATE_MODE) both
work fine.
Merge pull request #123 from iHiroakiKawai/libpcap_setnonblock

call pcap_setnonblock() after pcap_activate()