PDP8: Add device buffer flush capability and keep track of data written state in the device buffer.  Fixes #87

Finishing the last DECtape device which was missed when this functionality was added to pdp11_dt, pdp18b_dt and pdp8_dt
in commits: 2934112a7003bd31c135197a01b36d969e9c79ba and c9e8121c16ca7af2b79f45bba4b220e1c95983ba

Cleaned up the pdp11, pdp18b and other pdp8 implementations to make backporting cleaner.
Made sure that buffer flushing happens correctly when simulation returns to the sim> prompt.