select: don't embed fd_set inside iops structure

we only use it inside the loop, so this can be a loop-local variable

Change-Id: Ibcf728333751d8c00a4bc8951dbe961875734b0c
Reviewed-on: http://review.couchbase.org/33109
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>