Secure_link: fixed configuration inheritance.

The "secure_link_secret" directive was always inherited from the outer
configuration level even when "secure_link" and "secure_link_md5" were
specified on the inner level.


git-svn-id: svn://svn.nginx.org/nginx/trunk@5018 73f98a42-aea0-e011-b76d-00259023448c