More improvements in select.c

[jwt27]

Some more tweaks to poll()/select_s():

• If we get interrupted by a signal, but already have one or more ready sockets, that should not be an error.
• Yield should happen at the end of a loop (only if there is no reason to return yet). Also, no need to tcp_tick() twice in one loop.
• Then, I think the short-timeout hack for WinXP can be removed. If yield does stall, then the next loop will return some result.