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/dnsmasq.h

@@ -1314,6 +1314,7 @@ extern struct daemon {
   int dumpfd;
 #endif
   int max_procs;
+  uint max_procs_used;
 } *daemon;
 
 struct server_details {