* Use RequestURI when redirecting to canonical path.

Caddy may trim a request's URL path when it starts with the path that's
associated with the virtual host. This change uses the path from the request's
RequestURI when performing a redirect.

Fix issue #1327.

* Rename redirurl to redirURL.

* Redirect to the full URL.

The scheme and host from the virtual host's site configuration is used
in order to redirect to the full URL.

* Add comment and remove redundant check.

* Store the original URL path in request context.

By storing the original URL path as a value in the request context,
middlewares can access both it and the sanitized path. The default
default FileServer handler will use the original URL on redirects.

* Replace contextKey type with CtxKey.

In addition to moving the CtxKey definition to the caddy package, this
change updates the CtxKey references in the httpserver, fastcgi, and
basicauth packages.

* httpserver: Fix reference to CtxKey