mod_proxy_http: follow up to r1836588: avoid 100-continue responses from core.

When mod_proxy_http handles end-to-end "100 continue", it can't let
ap_http_filter() send its own interim response whenever the body is read.

So save/restore r->expecting_100 before/after handling the request, and use
req->expecting_100 internally (including to restore r->expecting appropriately).

While at it, add comments and debug logs about 100 continue handling, and
fill in missing APLOGNO()s from r1836588.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836648 13f79535-47bb-0310-9956-ffa450edef68