Pádraig, Sci-Fi, others,

I made an implementation of the requested feature. With the attached
patch applied the split command accepts a new optional "from" argument
for the --numeric-suffixes (aka -d) option. If this argument is
specified, the numeric suffix counts from this value, otherwise, like
before, it counts from 0.

I've tried to not impact the performance, to not break anything and to
respect the coding rules but feel free to comment this patch. I will
take into account whatever you may want.

Cheers,

Jérémy
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