fails:IO#close_on_exec= sets the close-on-exec flag if true
fails:IO#close_on_exec= sets the close-on-exec flag if non-false
fails:IO#close_on_exec= unsets the close-on-exec flag if false
fails:IO#close_on_exec= unsets the close-on-exec flag if nil
fails:IO#close_on_exec= ensures the IO's file descriptor is closed in exec'ed processes
fails:IO#close_on_exec= raises IOError if called on a closed IO
fails:IO#close_on_exec= returns nil
fails:IO#close_on_exec? returns true if set
fails:IO#close_on_exec? raises IOError if called on a closed IO
fails:IO#close_on_exec? returns true by default
