mod_proxy: Follow up to r1912245: ap_proxy_ prefix for extern functions.

mod_proxy: Follow up to r1912245: ap_proxy_ prefix for extern functions.

Even if they are not part of the API (not in mod_proxy.h) hence requires no
MMN bump, {get,set,increment_,decrement_}busy_count() being AP_PROXY_DECLARE()d
could name-collide with a third-party module's functions.

Rename them using the ap_proxy_ prefix, with an underscore after the verb for
for all of them too (for consistency), that is:
    ap_proxy_{get,set,increment,decrement}_busy_count()

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1913930 13f79535-47bb-0310-9956-ffa450edef68