[ruby-core:35527] using poll() for rb_io_wait_{read,writ}able()?

Hello, on some OSes it should be easy to swap out some current
users of select() for poll().  poll() works better on high-numbered
file descriptors.

I'm not sure how well poll() works for certain FD types (FIFO/anonymous
pipes/various socket types), but I know poll() on Linux works for
everything I've tried, so I might just enable it only under Linux until
other people can test it on other systems.

1. rb_io_wait_readable() + rb_io_wait_writable()
2. IO#wait
3. IO.select with a single object (common in my experience)

rb_thread_select() will probably always be around for backwards
compatibility.

IO.select for multiple IO objects might just be kept as-is for now.
IO.select seems to properly map multiple IO objects using the same FD
properly, which requires extra work and might not be worth it.

I can provide patches and try my best to not break compatibility
with other systems if folks are interested.

Thanks for reading!

-- 
Eric Wong