tail: prefer signed types to size_t, blksize_t

tail: prefer signed types to size_t, blksize_t

* src/tail.c (struct File_spec, xwrite_stdout, file_lines)
(pipe_lines, pipe_bytes, start_bytes, any_live_files)
(tail_forever, any_remote_file, any_non_remote_file)
(any_symlinks, any_non_regular_fifo, tailable_stdin)
(tail_forever_inotify, ignore_fifo_and_pipe, main):
Prefer a signed type to size_t, if possible.
Ordinarily this is idx_t, but use int when the value
must fit in int anyway.
(file_lines): Similarly for blksize_t, which had no business
being here anyway.
(main): Check for overflow in the oddball case where ptrdiff_t is
narrower than int.