Fixed and improved the "*_bind" directives of proxying modules.

The "proxy_bind", "fastcgi_bind", "uwsgi_bind", "scgi_bind" and
"memcached_bind" directives are now inherited; inherited value
can be reset by the "off" parameter.  Duplicate directives are
now detected.  Parameter value can now contain variables.


git-svn-id: svn://svn.nginx.org/nginx/trunk@5009 73f98a42-aea0-e011-b76d-00259023448c