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>

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