lighttpd-mod-auth has to be installed with lighttpd-mod-authn_file,
otherwise an error will appear even when auth.backend is not "plain".
(plugin.c.229) dlopen() failed for: /usr/lib/lighttpd/mod_authn_file.so Error loading shared library /usr/lib/lighttpd/mod_authn_file.so: No such file or directory
Signed-off-by: David Yang <mmyangfl@gmail.com>