Add new ap_reserve_module_slots/ap_reserve_module_slots_directive API,

necessary if a module (like mod_perl) registers additional modules later than the
EXEC_ON_READ phase.

Tested by: Torsten Foertsch <torsten foertsch gmx net>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1096569 13f79535-47bb-0310-9956-ffa450edef68