Added ability to log raw and PCAP-formatted packets in json alerts

Based on #922 (by MattCarothers) - adds ability to log PCAP-formatted packets in JSON alerts (fixes #988 - preserve history)

Both raw and PCAP-formatted packet are logged in Base64, in "pcap" and "packet_pcap" fields respectively.

"How to use" information added to suricata.yaml.in.

Also fixed a compile-time bug on old kernels withot ETHTOOL_GFLAGS define in ethtool.h.