Only add SSLProxyEngine in proxy mode and when SSL is enabled for the web (fixes #6269)

SSLProxyEngine is neither needed for http(s) redirects nor for non-SSL proxy configs.
Thus excluding it from these cases.

Based on a fix by Thom <thom@amsterdamtech.nl>

Author: jdsn

server/conf/vhost.conf.master

@@ -517,8 +517,12 @@
 		RewriteCond %{REQUEST_URI} !^/php-fcgi/
 		RewriteCond %{REQUEST_URI} !^<tmpl_var name='rewrite_target'>
 </tmpl_if>
+<tmpl_if name='use_proxy_protocol' op='==' value='y'>
+<tmpl_if name='ssl_enabled'>
 <tmpl_if name="rewrite_target_is_ssl" op="==" value="y">
 		SSLProxyEngine On
+</tmpl_if>
+</tmpl_if>
 </tmpl_if>
 
 		RewriteRule   ^/(.*)$ <tmpl_var name='rewrite_target'><tmpl_if name="rewrite_add_path" op="==" value="y">$1</tmpl_if>  <tmpl_var name='rewrite_type'>