numfmt: fix zero-padding placement for negative numbers (#11694)

* numfmt: fix zero-padding placement before sign for negative numbers

When using zero-padded format (e.g. %018.2f), the sign character is now
placed before the padding zeros, matching C printf behavior.

Fixes #11664

* numfmt: simplify sign handling with combined strip_prefix

tests/by-util/test_numfmt.rs

@@ -1428,7 +1428,6 @@ fn test_from_unit_fractional_precision_issue_11663() {
 // Zero-padded `--format` places padding zeros before the sign for negative
 // numbers; GNU (and C printf) puts the sign first.
 #[test]
-#[ignore = "GNU compat: see uutils/coreutils#11664"]
 fn test_zero_pad_sign_order_issue_11664() {
     new_ucmd!()
         .args(&["--from=none", "--format=%018.2f", "--", "-9869647"])