style: Introduce clang-format configurations for automated enforcement

[wmthornton-dev]

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.

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