quiet spurious wakeups for accept() in Thread* models

Under all MRI 1.8, a blocking Socket#accept Ruby method (needs
to[1]) translate to a non-blocking accept(2) system call that may
wake up threads/processes unnecessarily.  Unfortunately, we
failed to trap and ignore EAGAIN in those cases.

This issue did not affect Ruby 1.9 running under modern Linux
kernels where a _blocking_ accept(2) system call is not (easily,
at least) susceptible to spurious wakeups.  Non-Linux systems
running Ruby 1.9 may be affected.

[1] - using a blocking accept(2) on a shared socket with
      green threads is dangerous, as noted in
      commit ee7fe220ccbc991e1e7cbe982caf48e3303274c7
      (and commit 451ca6997b4f298b436605b7f0af75f369320425)

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