Code Robustness, --tty option and Build Simplification

• made the code more robust against programming, config and invocation errors:
  • included more checks
  • improved code structure (e.g. no vars in printf format strings)
  • improved error messages
  • introduced ci tests (shellcheck and nix build)
• added -t|--tty option to specify target terminal (requires sufficient permissions to grab)
• simplify build:
  • removed pandoc dependency by rewriting manual page in mdoc (semantic troff macros)
  • added nix derivation
  • docs: improved dependency and installation instructions