Fix PktSrc::Statistics returning bogus stats.

When reading from trace files, 'dropped' and 'link' fields are now
just zeroed.

When reading from an interface, the values filled in by pcap_stats()
are now only used when that function indicates success.

Closes #500.

testing/btest/Baseline/bifs.net_stats_trace/output

@@ -0,0 +1 @@
+[pkts_recvd=131, pkts_dropped=0, pkts_link=0]

testing/btest/bifs/net_stats_trace.test

@@ -0,0 +1,8 @@
+# Checks that accurate stats are returned when reading from a trace file.
+# @TEST-EXEC: bro -r $TRACES/wikipedia.trace >output %INPUT
+# @TEST-EXEC: btest-diff output
+
+event bro_done()
+	{
+	print net_stats();
+	}