MRI 1.8 thread fix to avoid blocking accept()

MRI 1.8 thread fix to avoid blocking accept()

Rainbows! commit ee7fe220ccbc991e1e7cbe982caf48e3303274c7

Under MRI 1.8, listen sockets do not appear to have the
nonblocking I/O flag on by default, nor does it set the
nonblocking I/O flag when calling #accept (but it does
when using #accept_nonblock, of course).

Normally this is not a problem even when using green threads
since MRI will internally select(2) on the file descriptor
before attempting a blocking (and immediately successful)
accept(2).

However, when sharing a listen descriptor across multiple
processes, spurious wakeups are likely to occur, causing
multiple processes may be woken up when a single client
connects.

This causes a problem because accept(2)-ing on multiple
threads/processes for a single connection causes blocking accepts in
multiple processes, leading to stalled green threads.

This is not an issue under 1.9 where a blocking accept() call
unlocks the GVL to let other threads run.