Fix regression on OS X from previous commit
Fix regression on OS X from previous commit

Attempting to put the PTY master in non-blocking mode with fcntl()
results in an error on OS X 10.8, with errno = ENOTTY (Inappropriate
I/O control operation). So for now, we will just rely on the behavior
of our main select() loop to ensure non-blocking operation.

markshroyer/nulltty,markshroyer/nulltty