atrf-txrx: added ability to record received frames in pcap format

- pcap.h: basic pcap file structure definitions
- atrf-txrx.c (usage, main): added option "-o file" to write received
  frames to a pcap file instead of displaying them
- atrf-txrx.c (receive): moved code to receive and display a single message
  into new function receive_message
- atrf-txrx.c (write_pcap_hdr, write_pcap_rec, receive_pcap): receive
  messages into a pcap file

tools/atrf-txrx/pcap.h

@@ -0,0 +1,49 @@
+/*
+ * pcap.h - Minimum pcap file definitions
+ *
+ * Written 2011 by Werner Almesberger
+ * Copyright 2011 Werner Almesberger
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+/*
+ * This header defines the few things we need to write files in the pcap
+ * format. The identifiers are the same as in the system's pcap/pcap.h, but
+ * the types have been standardized. Note that the timestamp parts have to be
+ * put separately, since "struct timeval" may be padded.
+ *
+ * The reason for having our own header instead of just using pcap/pcap.h is
+ * to avoid a build-dependency on libpcap.
+ */
+
+#ifndef PCAP_H
+#define	PCAP_H
+
+#include <stdint.h>
+#include <sys/time.h>
+
+
+#define	DLT_IEEE802_15_4	195
+
+struct pcap_file_header {
+	uint32_t magic;
+	uint16_t version_major;
+	uint16_t version_minor;
+	int32_t thiszone;
+	uint32_t sigfigs;
+	uint32_t snaplen;
+	uint32_t linktype;
+};
+
+struct pcap_pkthdr {
+	uint32_t ts_sec;
+	uint32_t ts_usec;
+	uint32_t caplen;
+	uint32_t len;
+};
+
+#endif /* !PCAP_H */