Proxy: strengthen syntax checking for some directives.

Proxy: strengthen syntax checking for some directives.

The "false" parameter of the proxy_redirect directive is deprecated.
Warning has been emitted since c2230102df6f (0.7.54).

The "off" parameter of the proxy_redirect, proxy_cookie_domain, and
proxy_cookie_path directives tells nginx not to inherit the
configuration from the previous configuration level.

Previously, after specifying the directive with the "off" parameter,
any other directives were ignored, and syntax checking was disabled.

The syntax was enforced to allow either one directive with the "off"
parameter, or several directives with other parameters.

Also, specifying "proxy_redirect default foo" no longer works like
"proxy_redirect default".