Building with a recent gcc-7 failed, so I wrote the attached patch. I
think we'll never have 2^31 command line arguments, so "unsigned int"
will ll be ok:

    maint: avoid md5sum.c warning from bleeding-edge gcc's -Wstrict-overflow

    Avoid this warning/error from gcc:
      src/md5sum.c:870:3: error: assuming signed overflow does not occur \
      when simplifying conditional to constant [-Werror=strict-overflow]
    * src/md5sum.c (main): Use an unsigned variable as the loop index,
    rather than optind.