MEDIUM: fd/poller: turn update_mask to group-local IDs

MEDIUM: fd/poller: turn update_mask to group-local IDs

From now on, the FD's update_mask only refers to local thread IDs. However,
there remains a limitation, in updt_fd_polling(), we temporarily have to
check and set shared FDs against .thread_mask, which still contains global
ones. As such, nbtgroups > 1 may break (but this is not yet supported without
special build options).