Clean up some proxy macros. Avoid the use of magic

numbers, and instead use pre-defined defines. Also,
ensure that usable workers have been initialized :)

Allocate a bit for hot standbys. Adjust so that
normal "usable" workers don't count these.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@420954 13f79535-47bb-0310-9956-ffa450edef68