style: Introduce clang-format configurations for automated enforcement

This commit introduces a formatting pipeline to enforce consistent
coding standards across the rsync codebase using clang-format.

The root `_clang-format` file is configured to enforce the Linux Kernel
(K&R) coding style, specifically adapted for rsync's historical baseline
and to account for developer habits:
- 8-character hardware tabs for all indentation
- 80-column line length limit
- K&R brace placement (same line for control statements, new line
  for function definitions)
- Function names and return types sharing the same line

Additionally, local `_clang-format` overrides have been added to the
`popt/` and `zlib/` vendored library directories. These files inherit
the root Linux structural rules but override the indentation to 4 spaces
to respect their specific upstream formatting requirements.

Usage:
To automatically apply the formatting rules and modify the files in-place,
run the following command from the root directory:

    find . -name "*.c" -o -name "*.h" | xargs clang-format -i

To automatically apply the formatting rules and modify a single file
in-place, run the following command from the root directory:

    clang-format -i <path/to/file>

To validate code compliance without modifying the files (for use
in pre-commit hooks or CI pipelines), run the dry-run command:

    find . -name "*.c" -o -name "*.h" | xargs clang-format --dry-run --Werror

To validate code compliance for a single file, without modification, run
the dry-run command:

   clang-format --dry-run --Werror <path/to/file>

This commit is designed to work in conjunction with .editorconfig files
but can be used standalone.

Additionally, a proposed STYLEGUIDE.md file is included in the root
directory for publishing where appropriate.

Signed-off by: Wayne Michael Thornton <wmthornton-dev@outlook.com>

Author: wmthornton-dev