csplit doesn't support precision

[sylvestre]

With ours:

$ echo > in
$ csplit -b '%0#6.3x' in 1
csplit: incorrect conversion specification in suffix

With GNU:

$ echo > in
$ /usr/bin/csplit -b '%0#6.3x' in 1
0
1

The regexp here:
https://github.com/uutils/coreutils/blob/main/src/uu/csplit/src/split_name.rs#L52
and the following code needs to be updated

Tested in GNU with tests/csplit/csplit.sh

[tertsdiepraam]

@marc-delgado-ferreres Glad you asked ChatGPT for us! Really helpful! Now please don't do that again.

Anyway, I think I just fixed the issue across these two PRs:

• csplit: use printf functionality from uucore #5958
• printf: pad unsigned numbers properly #5955

I'll verify that once those are merged and add some tests for it.