pcap/Source: Allow more than 32bit for link and dropped stats

The PktSrc::Stats object works with 64bit unsigned integers. Unfortunately,
libpcap's struct pcap_stat is using 32bit values and users have reported
the wrapping of these values being visible in their stats.log roughly every
7.5 hours (~160kpps).

This change moves tracking of link and drop counters into the PktSrc::Stats
object (like is done for received and bytes_received) and updates them
on a call to PcapSource::Statistics() with the difference to the
previous stats values to prevent the wrap from becoming visible to
script land.

This doesn't cover the case of the stats counters wrapping around multiple
times between two invocations of PktSrc::Statistics(). With the default
interval of 5 minutes for the stats script, this seems acceptable.

Closes #2791.

src/iosource/pcap/Source.h

@@ -43,6 +43,7 @@ private:
 	Stats stats;
 
 	pcap_t* pd;
+	struct pcap_stat prev_pstat = {0};
 	};
 
 	} // namespace zeek::iosource::pcap