pcapfile: make sure nanosecond timestamp resolution is retained

Previously, we were using trace_get_timeval() to get the
timestamp for a packet that is to be written, but obviously
this timestamp is only at microsecond resolution. Changed to
use trace_get_timespec() instead and the resulting nanosecs
are only then divided by 1000 if the output trace is going
to also use microsecond resolution.

lib/format_pcapfile.c

@@ -470,6 +470,12 @@ static int pcapfile_read_packet(libtrace_t *libtrace, libtrace_packet_t *packet)
     return sizeof(libtrace_pcapfile_pkt_hdr_t) + bytes_to_read;
 }
 
+#define IS_PCAP_NANOSECS(packet) \
+    (packet->trace && packet->trace->format && ( \
+                packet->trace->format->type == TRACE_FORMAT_PCAPFILE) && \
+                DATA(packet->trace) && \
+                trace_in_nanoseconds(&DATA(packet->trace)->header))
+
 static int pcapfile_write_packet(libtrace_out_t *out, libtrace_packet_t *packet)
 {
 
@@ -479,7 +485,7 @@ static int pcapfile_write_packet(libtrace_out_t *out, libtrace_packet_t *packet)
     }
 
     struct libtrace_pcapfile_pkt_hdr_t hdr;
-    struct timeval tv = trace_get_timeval(packet);
+    struct timespec ts = trace_get_timespec(packet);
     int numbytes;
     int ret;
     void *ptr;
@@ -516,9 +522,8 @@ static int pcapfile_write_packet(libtrace_out_t *out, libtrace_packet_t *packet)
             return -1;
         }
 
-        if (packet->trace && packet->trace->format && (
-                packet->trace->format->type == TRACE_FORMAT_PCAPFILE) &&
-                trace_in_nanoseconds(&DATA(packet->trace)->header)) {
+
+        if (IS_PCAP_NANOSECS(packet)) {
             pcaphdr.magic_number = MAGIC2;
         } else {
             pcaphdr.magic_number = MAGIC1;
@@ -533,8 +538,13 @@ static int pcapfile_write_packet(libtrace_out_t *out, libtrace_packet_t *packet)
         wandio_wwrite(DATAOUT(out)->file, &pcaphdr, sizeof(pcaphdr));
     }
 
-    hdr.ts_sec = (uint32_t)tv.tv_sec;
-    hdr.ts_usec = (uint32_t)tv.tv_usec;
+    hdr.ts_sec = (uint32_t)ts.tv_sec;
+    if (IS_PCAP_NANOSECS(packet)) {
+        hdr.ts_usec = (uint32_t)ts.tv_nsec;
+    } else {
+        hdr.ts_usec = (uint32_t)ts.tv_nsec / 1000;
+    }
+
     hdr.caplen = trace_get_capture_length(packet);
     if (hdr.caplen >= LIBTRACE_PACKET_BUFSIZE) {
         trace_set_err_out(out, TRACE_ERR_BAD_PACKET,