lighttpd: run as non-privileged user www-data

Previously the stock lighttpd configuration was patched to run as user root,
because we do not have a lighttpd user.
Patch it to use www-data instead, which is a user available in the default
skeleton, and is the same other webserver packages (like nginx) use.

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>