Add default redirect for profiling when enabled.

This commit adds a simple 303 redirect for the root of profile server so
launching it without a path automatically goes to the debug profiling
page.

btcd.go

@@ -48,6 +48,9 @@ func btcdMain() error {
 		go func() {
 			listenAddr := net.JoinHostPort("", cfg.Profile)
 			log.Infof("Profile server listening on %s", listenAddr)
+			profileRedirect := http.RedirectHandler("/debug/pprof",
+				http.StatusSeeOther)
+			http.Handle("/", profileRedirect)
 			log.Errorf("%v", http.ListenAndServe(listenAddr, nil))
 		}()
 	}