Support SIGUSR2 to stop and start logging for an existing server. Also

we currently only have two log levels so just use -v and -vv rather than
-v and -vvvv, and clarify the man page entry for -v.

log.c

@@ -62,12 +62,10 @@ log_open(const char *name)
 
 	if (log_level == 0)
 		return;
-
-	if (log_file != NULL)
-		fclose(log_file);
+	log_close();
 
 	xasprintf(&path, "tmux-%s-%ld.log", name, (long)getpid());
-	log_file = fopen(path, "w");
+	log_file = fopen(path, "a");
 	free(path);
 	if (log_file == NULL)
 		return;
@@ -76,6 +74,21 @@ log_open(const char *name)
 	event_set_log_callback(log_event_cb);
 }
 
+/* Toggle logging. */
+void
+log_toggle(const char *name)
+{
+	if (log_level == 0) {
+		log_level = 1;
+		log_open(name);
+		log_debug("log opened");
+	} else {
+		log_debug("log closed");
+		log_level = 0;
+		log_close();
+	}
+}
+
 /* Close logging. */
 void
 log_close(void)