MINOR: proxy: Only consider backend httpclose option for server connections

MINOR: proxy: Only consider backend httpclose option for server connections

For server connections, both the frontend and backend were considered to
enable the httpclose option. However, it is ambiguous because on client side
only the frontend is considerd. In addition for 2 frontends, one with the
option enabled and not for the other, the HTTP connection mode may differ
while it is a backend setting.

Thus, now, for the server side, only the backend is considered. Of course,
if the option is set for a listener, the option will be enabled if the
listener is the backend's connection.