mod_proxy: follow up to r1750392 and r1750474.

Restore PROXY_WORKER_IS_USABLE() check in ap_proxy_connect_backend(), we must
obviously (un)put backend in error state based on the result of the actual
connect(), and don't change it in ap_proxy_check_backend()...
APR_SUCCESS return by ap_proxy_check_backend(), i.e. a usable worker and an
established connection, is enough for modules to continue w/o calling
ap_proxy_connect_backend(), still.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1750508 13f79535-47bb-0310-9956-ffa450edef68