Provide ap_set_retained_data()/ap_get_retained_data() for preservation

of module state across unload/load.

The existing idiom used by modules to associate userdata with pglobal
doesn't work in the earliest phases of module execution.

(This does expose pglobal as an implementation detail, but it would be great 
to unexpose it if at all possible (but modules already have access to pglobal
at almost all stages of execution anyway).)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@758173 13f79535-47bb-0310-9956-ffa450edef68