Summary

Closes #16181. Prevents lightsleep being woken up every 64ms to service LWIP timers, when:

1. No netif is up, and
2. No TCP sockets are active

The TCP socket check may not be strictly necessary, but without ticking the tcp timer they won't ever time out by themselves.

It means that it's possible to do:

wlan.disconnect()
machine.lightsleep(5000)
wlan.connect(...)

... and get the full sleep time, but also possible to do

while some_condition():
    machine.lightsleep(5000)

... and Python will wake up every 64ms if Wi-Fi is active, but power consumption is still lower than it would be otherwise.

(Not massively lower, on RPI_PICO_W I measure 53mA idle at REPL with Wi-Fi associated, and 40mA in a while True: machine.lightsleep(1000) loop. There is probably potential to make this better by having lightsleep put the CYW43 driver into a lower power mode.)

This work was funded through GitHub Sponsors.

Testing

Important: This PR has only been tested cherry-picked onto the branch for #16454. It is marked as a draft until that PR is merged and it can be rebased.

• Ran multi_net and multi_wlan tests on RPI_PICO_W and RPI_PICO2_W. Same failures as reported against master.

Trade-offs and Alternatives

• There is some risk that some part of LWIP doesn't like being starved of timeouts for a long period after a network interface goes down. However, it looks like it will be OK - the main thing is TCP/IP socket timeouts and LWIP will keep ticking until these time out.
• Could make LWIP timer not a wake-up source, but this is quite difficult and does prevent using light sleep effectively with Wi-Fi.
• Could make LWIP timer wake-ups not return from lightsleep (i.e. the hardware wakes up, services LWIP, then goes back to sleep), but this is also quite fiddly to do (need to check the wake source each time).
• Semi-related: there is also a LWIP function sys_timeouts_sleeptime() which could be used to have the LWIP timer run at an exact time instead of at a constant tick rate. However, a number of LWIP "cyclic" timers run every 100ms right now so this change seems like it would be of limited benefit, it would add some code size, and the lwip tick callback is also used to poll the wiznet5k driver.