An invalid initial value for *read_pos would result in

An invalid initial value for *read_pos would result in
`tail -n0 -f FILE' and `tail -c0 -f FILE' doing what amounted to a
busy-wait rather than sleeping between iterations.  The bug manifests
itself only when tailing regular files that are initially nonempty.

(tail_bytes): Set *read_pos to new file offset after
each xlseek call.
(tail_lines): Likewise, after lseek calls.