mod_proxy: Improve the balancer member data shown

in mod_status when "ProxyStatus" is "On":
add "busy" count to html mode.

Backport of remaining parts of r1837588 from trunk
(only html mode changes, auto parts were already
backported).

Submitted by: rjung
Reviewed by: rjung, jim, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1839784 13f79535-47bb-0310-9956-ffa450edef68