Make error checking less zealous to handle devices like /dev/null

Make error checking less zealous to handle devices like /dev/null
which don't provide a non-blocking interface.

This is a short term "fix" which changes a half-lose to a half-win.
The thread that accesses a device that does not provide a non-blocking
interface will block for its time slice.

A medium term solution would be to use rfork. A long-term solution
would be some sort of kernel thread/SMP implementation.