Only call isatty() when we open our descriptor, and remember

Only call isatty() when we open our descriptor, and remember
the answer.

If we later get a descriptor exception from select(), we know
that it's a tty (isatty() returns 0 after the exception on a
tty) and remember to call modem_LogicalClose().

The upshot of it all is that descriptor exceptions dont leave
the tty locked any more.