shadow-core, tools: de-duplicate the clap-error boilerplate (#181)

[pierre-warnier]

Summary

Implements #181: hoists the copy-pasted clap-error handling out of all 12 tool crates into shadow-core.

New shared plumbing (shadow_core::cli, unit-tested):

• AlreadyPrinted(i32) — shared UError whose Display is empty (message already printed); carries only the exit code.
• parse_args(cmd, args, code) — parses argv, prints the clap error, returns Ok(None) for --help/--version (exit 0), and maps real parse failures through the tool's code function.

Per tool (chage, chfn, chpasswd, chsh, groupadd, groupdel, groupmod, newgrp, passwd, useradd, userdel, usermod):

• The 10-line try_get_matches_from match collapses to one let Some(matches) = parse_args(...)? else { return Ok(()); }.
• The AlreadyPrinted(i32) variant and its identical Display/code() arms are deleted from every enum; non-clap uses of the sentinel switch to the shared type.
• grpck/pwck (plain ?) are unchanged. Net: −180/+153 lines with 12 enums simplified.

Behavior preserved (verified, not assumed)

Ran the built binaries before/after:

Invocation	Exit code
passwd --help / --version	0, output byte-identical (passwd (uutils shadow-rs) 0.2.0)
passwd --bogus	6 (kind-dependent mapping kept)
passwd -l -u (conflict)	2
chpasswd --bogus	1
groupadd --bogus / missing arg	2

Notes:

• useradd's clap match had two arms that both returned 2 — collapsed to the constant (no behavior change).
• userdel's remaining variants all share the Cant prefix, tripping clippy::enum_variant_names; allowed with a comment since the names mirror userdel(8)'s documented exit-code names.

Test plan

• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace — green on Debian
• cargo test --workspace — green on Alpine (musl) and Fedora (59 passing suites each; matrix is push-gated on PRs)
• New unit tests for AlreadyPrinted (empty Display, code passthrough) and parse_args (success / error mapping / --version → Ok(None))

Closes #181.

[Copilot]

Pull request overview

This PR centralizes the previously copy‑pasted clap parse-error handling used across the shadow-rs tool crates into shadow_core::cli, replacing each tool’s local AlreadyPrinted(i32) sentinel and try_get_matches_from boilerplate with a shared parse_args helper while preserving exit-code behavior.

Changes:

• Added shadow_core::cli::{AlreadyPrinted, parse_args} (with unit tests) to standardize clap error printing and exit-code mapping.
• Updated the affected tools to use parse_args(...) and the shared AlreadyPrinted(...) instead of per-tool sentinel variants.
• Added clap and uucore as shadow-core dependencies to support the shared CLI plumbing.

Reviewed changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/uu/usermod/src/usermod.rs	Replaces local clap error boilerplate and sentinel with shadow_core::cli::parse_args / shared AlreadyPrinted.
src/uu/userdel/src/userdel.rs	Same refactor; also adds a clippy allow explaining intentional Cant* variant naming.
src/uu/useradd/src/useradd.rs	Collapses clap parsing boilerplate into parse_args and switches non-clap sentinel uses to shared AlreadyPrinted.
src/uu/passwd/src/passwd.rs	Uses parse_args with per-error-kind exit code mapping (2 vs 6), preserving GNU passwd behavior.
src/uu/newgrp/src/newgrp.rs	Replaces local clap boilerplate with shared parse_args and removes per-tool sentinel variant.
src/uu/groupmod/src/groupmod.rs	Replaces local clap boilerplate and sentinel with shared parse_args / AlreadyPrinted.
src/uu/groupdel/src/groupdel.rs	Replaces local clap boilerplate and sentinel with shared parse_args / AlreadyPrinted.
src/uu/groupadd/src/groupadd.rs	Replaces local clap boilerplate and sentinel with shared parse_args / AlreadyPrinted.
src/uu/chsh/src/chsh.rs	Replaces local clap boilerplate and sentinel with shared parse_args / AlreadyPrinted.
src/uu/chpasswd/src/chpasswd.rs	Replaces local clap boilerplate and sentinel; updates unit tests to assert shared AlreadyPrinted behavior.
src/uu/chfn/src/chfn.rs	Replaces local clap boilerplate and sentinel with shared parse_args / AlreadyPrinted.
src/uu/chage/src/chage.rs	Replaces local clap boilerplate and sentinel; updates unit tests to assert shared AlreadyPrinted behavior.
src/shadow-core/src/cli.rs	Adds shared AlreadyPrinted and parse_args plus unit tests; expands module docs to cover shared CLI plumbing.
src/shadow-core/Cargo.toml	Adds clap and uucore dependencies needed by the new shared CLI utilities.
Cargo.lock	Records the new shadow-core dependency edges on clap and uucore.

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