su-common.c: prefer /etc/default/su over login.defs

su-common.c: prefer /etc/default/su over login.defs

su(1) documentation says:
       /etc/default/su  command specific logindef config file
       /etc/login.defs  global logindef config file

It indirectly indicates that /etc/default/su should take precedence
over /etc/login.defs.

But the reverse is true. It is not possible to define ENV_PATH in
/etc/login.defs and then make su specific customization in
/etc/default/su. We need to change read order to match the documented
behavior.

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>