tail: track errno more accurately

tail: track errno more accurately

This matters only in some obscure cases hard to test for.
* src/tail.c (file_lines, pipe_lines, pipe_bytes, start_bytes)
(start_lines, tail_bytes, tail_lines, tail): New return
convention, which reports errno.  All callers changed.
(recheck): Don’t lose track of errno if a regular file is
replaced by a symlink.
(get_file_status): Set errno to 0 on success.
(tail_file): Be more careful about f->errnum.
It is now -1 only if the failure was not due to
a system call failing.