shuf: prefer fwrite over fputs and fputc

shuf: prefer fwrite over fputs and fputc

On an AMD Ryzen 7 3700X running GNU/Linux:

    $ timeout 30 taskset 1 ./src/shuf-prev \
        -r -i 1000000-1000000 | pv -r > /dev/null
    [ 302MiB/s]
    $ timeout 30 taskset 1 ./src/shuf \
        -r -i 1000000-1000000 | pv -r > /dev/null
    [ 434MiB/s]

* src/shuf.c (print_number): New function.
(write_permuted_numbers, write_random_numbers): Use it.
* tests/shuf/shuf.sh: Add a test case to run 'shuf -i' with varying
numbers of digits to check that the string conversion is correct.