Take proxy server from internet settings, and pass to cURL.
Allow forwarding authentication as well (explicitely setting
user/password is still missing).

Change-Id: I19a6c9057a11a5911a6117f71060d3f386953602
Reviewed-on: https://gerrit.libreoffice.org/51621Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>