MXS-2197: Make SERV_LISTENER a C++ class

The class is still mostly the same as the old C version but it now uses
std::string instead of char pointers. Changed configuration default values
so that the parameters passed to the listener allocation are always valid.

server/modules/protocol/HTTPD/httpd.cc

@@ -206,7 +206,7 @@ static int httpd_read_event(DCB* dcb)
     /** If listener->authenticator is the default authenticator, it means that
      * we don't need to check the user credentials. All other authenticators
      * cause a 401 Unauthorized to be returned on the first try. */
-    bool auth_ok = strcmp(httpd_default_auth(), dcb->listener->authenticator) == 0;
+    bool auth_ok = httpd_default_auth() == dcb->listener->authenticator;
 
     /**
      * Get the request headers