Introduce ap_(get|set)_core_module_config() functions/macros and use them

everywhere.

We know that the core module has module_index 0. Therefore we can save
some pointer operations in ap_get_module_config(cv, &core_module) and
ap_set_module_config(cv, &core_module, val). As these are called rather often,
this may actually have some (small) measurable effect.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1132781 13f79535-47bb-0310-9956-ffa450edef68