PAM: add 'uid_min' and 'uid_max' options for changing the uid range

Instead of a fixed >=1000 check, allow the configuration to override
the minimum UID and add a maximum one as well. While here, add the
uid range check to the authenticate method as well, and fix the return
in the chauthtok method (seems very wrong to report success when we've
done absolutely nothing).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Felix Dörre <felix@dogcraft.de>
Signed-off-by: Val Packett <val@packett.cool>
Closes #14834