shuf: avoid locking standard output when using --input-range

shuf: avoid locking standard output when using --input-range

Here is the throughput before this patch:

    # write_permuted_numbers
    $ ./src/shuf-prev -i 0-100000000 | pv -r > /dev/null
    [ 153MiB/s]

    # write_random_numbers
    $ timeout 10 ./src/shuf-prev -i 0-100000 -r | pv -r > /dev/null
    [78.6MiB/s]

Here is the throughput after this patch:

    # write_permuted_numbers
    $ timeout 10 ./src/shuf -i 0-100000000 | pv -r > /dev/null
    [ 308MiB/s]

    # write_random_numbers
    $ timeout 10 ./src/shuf -i 0-100000 -r | pv -r > /dev/null
    [ 196MiB/s]

* NEWS: Mention the performance improvement.
* src/shuf.c (write_permuted_numbers, write_random_numbers): Prefer
fputs and fputc which may be unlocked over printf which locks standard
output.