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.c

@@ -39,6 +39,7 @@ const char * metric_names[] = {
     "leases_pruned_4",
     "leases_allocated_6",
     "leases_pruned_6",
+    "tcp_connections",
 };
 
 const char* get_metric_name(int i) {