rtlplayground: keep the STP and TCP timers on the system tick

stp_timers() ran every fourth pass of the main loop and stp_clock counted
passes, which only measures time while a pass is one tick long. It never
quite was: a pass is as long as the packets it handles, and #482 now takes
up to four of them, so under an IPv4 stream a pass is about 16 ms and STP
time runs at a third of its rate, with hellos 6 s apart and a root aged out
after a minute. Since #483 the loop also stops sleeping while packets wait,
so under a flood of cheap frames passes come faster than ticks and STP time
runs fast instead, shortening the forward delay in exactly the situation a
loop produces. uip_periodic() had the same problem from the other side: it
ran once per pass, so the retransmit timer sped up with the loop.

Both now go by the tick counter the timer interrupt keeps. The STP block
catches up on ticks a long pass missed, at most eight steps at a time, and
handle_tx() runs its periodics once per tick at most. One byte of xdata
more, 48 bytes of code.

A host test drives the gate with one pass every 5 ms, every 16 ms and
every 0.6 ms for ten seconds each; the STP clock lands on 500 steps in
all three, where counting passes gave 156 and 4166. A pass of 1.3 s loses
time rather than firing 65 steps at once, and the arithmetic survives the
tick counter wrapping.

rtl837x_common.h

@@ -7,6 +7,14 @@
 
 #define SYS_TICK_HZ 200
 
+/* Timers of the main loop count system ticks, not loop passes: a pass is as
+ * long as the packets it handles, and since #483 the loop no longer sleeps
+ * while packets wait, so passes can outrun the tick as well as lag it. `last`
+ * is the low byte of ticks when the timer last ran; the byte arithmetic wraps
+ * every 256 ticks, so a pass longer than 1.28 s loses time rather than
+ * running the timer 64 times to catch up. */
+#define TICKS_DUE(last, period)	((uint8_t)((uint8_t)ticks - (last)) >= (period))
+
 #define CPU_PORT        9
 #define NUL			'\0'