Fix pcap packet acquisition methods

Fix pcap packet acquisition methods

Fix pcap packet acquisition methods passing 0 to pcap_dispatch.
Previously they passed the packet pool size, but the packet_q_len
variable was now hardcoded at 0.

This patch sets packet_q_len to 64. If packet pool is empty, we fall
back to direct alloc. As the pcap_dispatch function is only called
when packet pool is not empty, we alloc at most 63 packets.