easy way to determine if a stream or fd is seekable

[perryh at pluto.rain.com]

"Dieter BSD" <dieterbsd at engineer.com> wrote:
> IIRC some tape drives can seek, while others cannot.
> Vague memories that it is supposed to be possible to put a
> filesystem on a DECtape and mount the filesystem.

Back in the Bell Labs 6th Edition days, it was possible to put
a filesystem on a _9-track magtape_ and mount it, although such
a mount had to be read-only since writing to a 9-track would
effectively delete any blocks following the one written.
I've done it.  Access was painfully slow (no surprise), but
it did work.

DECtape _could_ be updated in place.  It was effectively the
linear equivalent of a floppy disk (before floppy disks existed).
Sequential access was slow.  Random access was very slow.