Avoid a race condition in `tail -f' described by Ken Raeburn in
http://mail.gnu.org/archive/html/bug-textutils/2003-05/msg00007.html
* src/tail.c (file_lines): Add new parameter, *read_pos, and set it.
- (pipe_lines): Likewise.
- (pipe_bytes): Likewise.
- (start_bytes): Likewise.
- (start_lines): Likewise.
- (tail_bytes): Likewise.
- (tail_lines): Likewise.
- (tail): Likewise.
+ (pipe_lines, pipe_bytes, start_bytes, start_lines): Likewise.
+ (tail_bytes, tail_lines, tail): Likewise.
(tail_file): Use the new `read_pos' value as the size,
rather than stats.st_size from the fstat call.