Revert to LIBNET_LINK where LIBNET_NONE is not available

Old versions of libnet do not support LIBNET_NONE; on those, use
LIBNET_LINK, which has the unfortunate side-effect of needlessly opening
a raw socket, which requires root privileges.  Also, the type of link
layer encapsulation generated by this libnet instance will depend on the
active network interface chosen by libnet for the raw socket, which may
result in uncommon link encapsulations being used for the PCAP files
written.

Issue:		droe#232

log.c

@@ -1131,7 +1131,18 @@ log_content_pcap_preinit(const char *pcapfile)
 {
 	char errbuf[LIBNET_ERRBUF_SIZE];
 
+#ifdef LIBNET_NONE
 	content_pcap_libnet = libnet_init(LIBNET_NONE, NULL, errbuf);
+#else /* !LIBNET_NONE */
+	/* Old versions of libnet do not support LIBNET_NONE; on those, we use
+	 * LIBNET_LINK, which has the unfortunate side-effect of needlessly
+	 * opening a raw socket, which requires root privileges.  Also, the
+	 * type of link layer encapsulation generated by this libnet instance
+	 * will depend on the active network interface chosen by libnet for the
+	 * raw socket, which may result in uncommon link encapsulations being
+	 * used for the PCAP files written. */
+	content_pcap_libnet = libnet_init(LIBNET_LINK, NULL, errbuf);
+#endif /* !LIBNET_NONE */
 	if (content_pcap_libnet == NULL) {
 		log_err_printf("Failed to init pcap libnet: %s\n", errbuf);
 		return -1;