The selinux and fts-sys crates should only be required on Linux platforms.
This change moves these dependencies from the unconditional [dependencies]
section to [target.'cfg(target_os = "linux")'.dependencies] in both
chcon and runcon Cargo.toml files.

Without this fix, macOS builds fail because Cargo attempts to resolve
Linux-only dependencies on non-Linux platforms, even though the Rust
code itself is properly gated with #[cfg(target_os = "linux")].

Changes:
- chcon: Move selinux and fts-sys to Linux-only dependencies
- runcon: Move selinux to Linux-only dependencies
- runcon: Move selinux feature in uucore to Linux-only section

This ensures that on non-Linux platforms (macOS, Windows):
- chcon and runcon build as stubs with empty main()
- No SELinux libraries are linked
- The workspace builds successfully without SELinux dependencies

On Linux platforms:
- All SELinux functionality remains unchanged
- selinux and fts-sys are included as before
- No regression in behavior

Resolves the macOS CI build failure (exit code 101) in PR uutils#8795.

…ssages

Address maintainer feedback to verify SELinux utilities build as stubs
on non-Linux platforms (macOS, Windows).

Changes:
- Add dedicated GitHub Actions job 'test_selinux_stubs' to verify
  chcon/runcon build successfully on macOS and Windows
- Enhance stub implementations with user-friendly error messages
  instead of silent success
- Test binary creation, compilation, and workspace integration

Benefits:
- Ensures cross-platform builds remain functional in CI
- Provides clear feedback to users on unsupported platforms
- Matches GNU coreutils behavior (error message + exit code 1)
- Prevents confusion when utilities are accidentally invoked

Test coverage:
- Builds uu_chcon and uu_runcon explicitly on macOS/Windows
- Verifies stub binaries are created
- Validates cargo check succeeds
- Ensures full workspace builds with stubs present

Addresses feedback from @sylvestre in PR uutils#8795

Gate SELinux functionality to Linux-only and provide stub implementations
for chcon/runcon on non-Linux platforms to maintain cross-platform builds.

Changes:
- Gate all SELinux code with target_os = "linux" checks
- Add stub main() for chcon/runcon on non-Linux with user-friendly errors
- Add CI job to verify stubs build correctly on macOS and Windows
- Update ls to check both selinux feature AND target_os

Benefits:
- Fixes build failures on macOS/Windows (uutils#8581, uutils#7996, uutils#7695, uutils#6491)
- Maintains workspace buildability across all platforms
- Provides clear error messages instead of silent failures
- Prevents accidental SELinux usage on unsupported platforms

CI Testing:
- New 'Build/SELinux-Stubs (Non-Linux)' job tests macOS and Windows
- Verifies stub binaries are created and compilation succeeds
- Validates full workspace builds with stubs present

Addresses maintainer feedback in PR uutils#8795