• (libeio) work around a Linux (and likely FreeBSD and other
  kernels) bug where sendfile would not transfer all the requested
  bytes on large transfers, using a heuristic.
  
• buggy sendfile caused aio_move/copy to sometimes fail for big
  files (fortunately it checks that the whole file has been
  transferred...)
  
• use libecb for higher performance and higher portability.
  
• (libeio) disable sendfile on darwin, it's too broken.
  
• disable fork tests on !linux, as only linux supports mixing
  pthread and fork in perl.
  
• document the fact that fork doesn't work anymore when using
  this module, on anything !GNU/Linux.
  
• increase timeout in t/04_fork.t, as too many CPAN-tester setups
  run on a 0.1MHz cpu. Or so.