SysN test != V7 test

[Dave Martindale]

Re: test -w /dev/rmt0 writes a tape mark on tape in v7

Interesting.  You can probably still read much of the data on the tape
by skipping over the tape mark.  However, if the tape was originally
written at one density, and the tape mark gets written at a different
density, you won't be able to read anything at all, since drives which
handle multiple recording densities usually look at an identification
burst (or lack of it) at the beginning of the tape to determine the density
of the rest of the tape.