make config file override grant database

make config file override grant database

Even though a caller may have an entry in the grant database (and as
such will see POLKIT_RESULT_YES), change the behavior such that this
is no longer honored unless the config file specifies the result
POLKIT_RESULT_ONLY_VIA_[SELF|ADMIN]_AUTH_{,KEEP_SESSION|KEEP_ALWAYS}.

E.g. this allows the sysadmin to specify things like POLKIT_RESULT_NO
in the config file and that will now make existing grants
useless. This behavior is a lot more natural.