[php-src] PR #24249: Rename FPM's apache_request_headers() implementation to fpm_request_headers

Pull Request: https://github.com/php/php-src/pull/24249 Author: mnapoli The CLI SAPI (for its built-in web server) and FPM both implement apache_request_headers() with PHP_FUNCTION(apache_request_headers), so both define the C symbol zif_apache_request_headers. That makes it impossible to link the two SAPIs into the same executable: the link fails with a duplicate symbol. This renames FPM's implementation to fpm_request_headers (the name suggested in the review of #23558), and maps apache_request_headers() and getallheaders() to it with @implementation-alias. There is no behavior change: the PHP functions are unchanged. ### Why It is needed by #24192, which links the CLI into php-fpm ([RFC: Single binary for CLI and FPM](https://wiki.php.net/rfc/single-binary-cli-fpm)). The rename doesn't depend on the RFC, so it can be merged on its own. This keeps #24192 smaller, and helps builds that link both SAPIs together outside of php-src. ### Tests sapi/fpm/tests passes, including getallheaders.phpt.