Upstream: consistently initialize explicit upstreams.

Upstream: consistently initialize explicit upstreams.

When an upstream{} block follows a proxy_pass reference to it,
such an upstream inherited port and default_port settings from
proxy_pass.  This was different from when they came in another
order (see ticket #1059).  Explicit upstreams should not have
port and default_port in any case.

This fixes the following case:

	server { location / { proxy_pass http://u; } ... }
	upstream u { server 127.0.0.1; }
	server { location / { proxy_pass https://u; } ... }

but not the following:

	server { location / { proxy_pass http://u; } ... }
	server { location / { proxy_pass https://u; } ... }
	upstream u { server 127.0.0.1; }