Change forthcoming for Apache 2.4 support

[Cantor, Scott]

As a heads up, I have a fix going into 2.5.2 for this SP issue:

https://issues.shibboleth.net/jira/browse/SSPCPP-570

I didn't notice the implications of the collision with valid-user. It worked before because every module got a chance to see every require rule, so I wasn't preventing valid-user from being handled by other modules.

The most exedient fix I could see is described in the bug, basically I need to deprecate "require valid-user" for sites that mix in other modules and add a "require shib-session" rule that has the current semantics.

To avoid a breaking change, sites mixing modules will have to explicitly add a server level command, "ShibCompatValidUser On", to allow valid-user to return to its non-SP semantics. It's added to the apache24.config sample, set Off.

I have backported part of this to Apache 2.2/2.0 , and will change the documentation to reflect that shib-session should be used going forward.

-- Scott