Add information on process-forking for TCP connections to metrics.

Add the relevant information to the metrics and to the output of
dump_cache() (which is called when dnsmasq receives SIGUSR1).
Hence, users not collecting metrics will still be able to
troubleshoot with SIGUSR1. In addition to the current usage,
dump_cache() contains the information on the highest usage
since it was last called.

src/metrics.h

@@ -38,6 +38,7 @@ enum {
   METRIC_LEASES_PRUNED_4,
   METRIC_LEASES_ALLOCATED_6,
   METRIC_LEASES_PRUNED_6,
+  METRIC_TCP_CONNECTIONS,
   
   __METRIC_MAX,
 };