MAJOR: pollers: rely on fd_reregister_all() at boot time

MAJOR: pollers: rely on fd_reregister_all() at boot time

The poller-specific thread init code now uses that new function to
safely register boot events. This ensures that we don't register an
event for another group and that we properly deal with parallel
thread startup.

It's only done for thread-aware pollers, there's no point in using
that in poll/select though that should work as well.