Better handling of late upstream creation.

Better handling of late upstream creation.

Configuration with duplicate upstream blocks defined after first use, i.e.
like

    server {
        ...
        location / {
            proxy_pass http://backend;
        }
    }

    upstream backend { ... }
    upstream backend { ... }

now correctly results in "duplicate upstream" error.

Additionally, upstream blocks defined after first use now handle various
server directive parameters ("weight", "max_fails", etc.).  Previously
configuration like

    server {
        ...
        location / {
            proxy_pass http://backend;
        }
    }

    upstream backend {
        server 127.0.0.1 max_fails=5;
    }

incorrectly resulted in "invalid parameter "max_fails=5"" error.