Implement expr lookup in mod_proxy_hcheck for

variables whose names start with "HC_" and for
the new function hc().

Currently only HC_BODY and hc(body) are supported.
Both return the saved body of the health check
response to be used in an expr that decides about
success of a check.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1726038 13f79535-47bb-0310-9956-ffa450edef68