easy way to determine if a stream or fd is seekable

[Dieter BSD]

> lseek() on a tape drive does not return an error, nor does it
> actually do anything.

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.

It might be that FreeBSD doesn't currently support seeking
on a tape, but we shouldn't paint ourselves into a corner
by assuming that it is fundimentally impossible.