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.

proc.c

@@ -265,3 +265,9 @@ proc_kill_peer(struct tmuxpeer *peer)
 {
 	peer->flags |= PEER_BAD;
 }
+
+void
+proc_toggle_log(struct tmuxproc *tp)
+{
+	log_toggle(tp->name);
+}