script: fix EOF problems

script: fix EOF problems

* remove STDIN from poll() if:

  * STDIN already closed -- poll returns POLLHUP, for example:

		echo "date" | script

  * detect EOF as returned by read(), for example:

		script -c "echo Hello" < /dev/null

* don't write to master when there is still something to read from
  slave (it means shell is not initialized yet or busy)

Signed-off-by: Karel Zak <kzak@redhat.com>