Comprehensive repository improvements: fix type bug, reduce package size, and improve maintainability

[Copilot]

This PR addresses the requested repository review with improvements prioritized as follows:

🔧 Critical Bug Fix

• Fixed type error in get_user_id_of(): Changed incorrect return type from libc::gid_t to libc::uid_t. This was a potentially serious bug that could cause incorrect behavior when setting user IDs.

📦 Package Size Reduction

• Made log dependency optional: The log crate is now an optional dependency behind the logging feature (enabled by default)
• Users can opt out with: drop-root = { version = "1.0", default-features = false }
• This reduces the dependency footprint for users who don't need logging

🛠️ Code Quality & Maintainability

• Fixed all clippy warnings: Updated format strings to use modern inline syntax ({variable} instead of {}, variable)
• Improved error messages: Fixed inconsistent error message for setgroups operation
• Removed unnecessary empty line after outer attribute in lib.rs

🔄 CI/CD Improvements

• Updated GitHub Actions: Replaced deprecated actions/checkout@v2 and actions-rs/toolchain@v1 with modern alternatives
• Added feature testing: CI now validates the crate works both with and without the logging feature
• Enhanced linting: Added clippy checks to CI pipeline

📚 Documentation

• Updated README: Added documentation for the new optional logging feature
• Fixed doctest: Removed unnecessary fn main() wrapper in README example

Testing

All existing tests pass, and the changes are backward compatible. The crate now:

• ✅ Compiles with default features (logging enabled)
• ✅ Compiles with --no-default-features (logging disabled)
• ✅ Passes all clippy lints
• ✅ Maintains the same public API

The changes are minimal and surgical, focusing on fixing the critical bug while improving code quality and reducing optional dependencies.

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[jhg]

You can reduce uploaded size of package if in Cargo.toml you use something like:

exclude = [
    ".github/**",
    ".vscode/**",
    ".idea/**",
    "tests/**", # To reduce the package size, as tests are not needed to use it.
]

or:

exclude = [".github/**", ".vscode/**", "benches/**", "Cargo.lock", "LICENSE", ".gitignore"]

[jhg]

It's ok, but could be nice also run tests without logging feature.

[Copilot]

Great idea! I've added a dedicated step to run tests without the logging feature. The CI now includes both cargo check --no-default-features and cargo test --no-default-features to ensure the crate works properly in both configurations. (3571bbf)

[Copilot]

You can reduce uploaded size of package if in Cargo.toml you use something like:

...

Good suggestion! I've expanded the exclude list in Cargo.toml to include .vscode/**, .idea/**, tests/**, benches/**, and Cargo.lock to further reduce the package upload size. This will help keep the published crate smaller by excluding development files. (3571bbf)