Referer: "server_names" parsing deferred to merge phase.

This allows to approach "server_name" values specified below the
"valid_referers" directive when used within the "server_names" parameter, e.g.:

    server_name  example.org;
    valid_referers server_names;
    server_name  example.com;

As a bonus, this fixes bogus error with "server_names" specified several times.