update all dependencies to the latest version + various small fixes

[decathorpe]

I'm the maintainer of the package for silver in Fedora Linux, and it's getting increasingly difficult to keep it in the distro because the dependencies are starting to be quite outdated. This PR should fix most of that (I hope) :)

It's split into three commits:

1. update dependencies (including patches) to match the current state in Fedora:
   • update clap to stable v3, use the "derive" feature instead of depending on clap_derive directly
   • update confy to stable v0.5 to replace the git dependency
   • update git2 to v0.18 to address multiple security vulnerabilities in git2 / libgit2-sys / bundled libgit2 prior to v0.18.2 of the git2 crate
   • update rust-ini to v0.17
   • update sysinfo to v0.23
   • port from unmaintained + unsound users v0.11 to uzers drop-in replacement
2. update dependencies (including patches) to the latest available versions:
   • update clap to v4
   • update confy to v0.6
   • update dirs to v5
   • update rust-ini to v0.20
   • update shellexpand to v3 (this required small code changes)
   • update sysinfo to v0.30
3. address all warnings raised by current rust / clippy

Feel free to cherry-pick any of these changes individually, I don't mind if this PR does not get merged as-is (or at all), but I already did most of the work on this, so I thought I might as well open a PR with my changes.