mod_proxy_ajp: Forward SSL protocol name (SSLv3, TLSv1.1 etc.) as a
request attribute to the backend. The name is taken from
the SSL_PROTOCOL env var. Recent Tomcat versions will extract
it and provide it as a servlet request attribute named
"org.apache.tomcat.util.net.secure_protocol_version". 

The change is compatible with existing AJP13, because we forward
the protocol name as a normal named attribute, not with a new
byte abbreviated attribute name.

Submitted by: rjung
Reviewed by: rjung, ylavic, covener
Backported by: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1662076 13f79535-47bb-0310-9956-ffa450edef68