methods: read config in most to least specific order

The implementation of the generic config fallback did the fallback in
the wrong order so that the least specific option wasn't the last value
picked but in fact the first one… doh!

So in the bugreports case http -> https -> http::<hostname> ->
https::<hostname> while it should have been the reverse as before.

Regression-In: 30060442025824c491f58887ca7369f3c572fa57
Closes: 834642

methods/aptmethod.h

@@ -86,10 +86,10 @@ public:
    }
    std::string ConfigFind(char const * const postfix, std::string const &defValue) const APT_NONNULL(2)
    {
-      for (auto && name: methodNames)
+      for (auto name = methodNames.rbegin(); name != methodNames.rend(); ++name)
       {
 	 std::string conf;
-	 strprintf(conf, "Acquire::%s::%s", name.c_str(), postfix);
+	 strprintf(conf, "Acquire::%s::%s", name->c_str(), postfix);
 	 auto const value = _config->Find(conf);
 	 if (value.empty() == false)
 	    return value;

test/integration/test-bug-623443-fail-on-bad-proxies

@@ -29,5 +29,6 @@ testfailure apt update
 unset http_proxy
 testsuccess grep 'Unsupported proxy configured' rootdir/tmp/testfailure.output
 
-
-
+echo 'Acquire::http::Proxy "foo://example.org";
+Acquire::https::Proxy "DIRECT";' > rootdir/etc/apt/apt.conf.d/proxy.conf
+testsuccess apt update