critical(hangs):IO.popen allows the io to be closed inside the block
unstable(JRUBY-4171,linux,intermittent failure):IO.popen writes to a write-only pipe
fails:IO.popen starts returns a forked process if the command is -
windows(JRUBY-4701):IO.popen returns the IO if no block given
windows(JRUBY-4814):IO.popen reads from a read-only pipe
