port: add pcap file source

port: add pcap file source

Originally, source ports in librte_port is an input port used as packet
generator. Similar to Linux kernel /dev/zero character device, it
generates null packets. This patch adds optional PCAP file support to
source port: instead of sending NULL packets, the source port generates
packets copied from a PCAP file. To increase the performance, the packets
in the file are loaded to memory initially, and copied to mbufs in circular
manner. Users can enable or disable this feature by setting
CONFIG_RTE_PORT_PCAP compiler option "y" or "n".

Signed-off-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>