add multiple domains reverse SSL proxy support

Add support for a reverse proxy that handles multiple domains via different
web roots (https://proxy.tld/domain.tld/owncloud) and only forwards SSL
connections unencrypted to the web server.

As the reverse proxy web root is transparent for the web server the
REQUEST_URI and SCRIPT_NAME need manual adjustments. This patch replace
the direct use of this _SERVER variables with function calls. Additionally
it adds a Sabre request backend that extends the Sabre_HTTP_Request to use
the same functions.