select: Don't allow unwanted/leftover fds being returned.

select: Don't allow unwanted/leftover fds being returned.

The root cause is that the lwp_kqueue_serial will wrap pretty quickly,
6 seconds on my laptop, if the select(2) is polling, either due to heavy
workload or 0 timeout.  The POC test:
https://leaf.dragonflybsd.org/~sephe/select_wrap.c

Fixing this issue by saving the original fd_sets and do additional
kevent filtering before return the fd to userland.

poll(2) suffers the similar issue and will be fixed in later commit.

Reported-by: many