tail -c +N would perform an extra read after encountering EOF

tail -c +N would perform an extra read after encountering EOF
[this change is analogous (bytes vs. lines) to the one of 2002-01-27]

(start_bytes): Detect EOF, inform caller.
(tail_bytes): Upon EOF in start_bytes, return immediately.
(file_lines): Reorganize to use memrchr rather than an explicit loop.
Adapt to new safe_read ABI.