Track state to determine if the associated TTY device node has been used.

Track state to determine if the associated TTY device node has been used.

It turns out our old TTY layer (and other implementations) block when
you read() on a PTY master device of which the slave device node has not
been opened yet. Our new implementation just returned 0. This caused
applications like telnetd to die in a very subtle way (when child
processes would open the TTY later than the first call to select()).

Introduce a new flag called PTS_FINISHED, which indicates whether we
should block or bail out of a read() or write() occurs.

Reported by:	Claude Buisson <clbuisson orange fr>