uniq: avoid locking standard output when printing line counts with -c

uniq: avoid locking standard output when printing line counts with -c

This increases throughput on systems with unlocked stdio functions.
E.g., 2.5x in the following case:

    $ timeout 1m taskset 1 shuf -r -i 0-100 \
        | taskset 2 uniq -c \
        | taskset 3 pv -r > /dev/null
    [ 100MiB/s]
    $ timeout 1m taskset 1 shuf -r -i 0-100 \
        | taskset 2 uniq -c \
        | taskset 3 pv -r > /dev/null
    [ 246MiB/s]

* src/uniq.c: Include assure.h.
(writeline): Convert the value to a string by hand and use fwrite, which
may be unlocked, instead of printf.
* tests/uniq/uniq-c-width.sh: New test to verify the padding logic.
* tests/local.mk (all_tests): Add the test case.
* NEWS: Mention the improvement.