Merge branch 'jc/max-io-size-and-ssize-max'

Our default I/O size (8 MiB) for large files was too large for some platforms with smaller SSIZE_MAX, leading to read(2)/write(2) failures. * jc/max-io-size-and-ssize-max: xread/xwrite: clip MAX_IO_SIZE to SSIZE_MAX