Add 's' format tag to mod_headers to allow access to mod_ssl variables

Add 's' format tag to mod_headers to allow access to mod_ssl variables
without the overhead of SSLOptions +StdEnvVars/+ExportCertData; also
ensure that any multi-line envvars are unwrapped onto a single line.

* modules/metadata/mod_headers.c (header_request_ssl_var, unwrap_header,
header_post_config): New functions.
(header_pre_config): Register the 's' tag type.
(register_hooks): Add the post_config hook.

PR: 23223

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@102809 13f79535-47bb-0310-9956-ffa450edef68