Some auth plugins used to have a mix of the legacy style of plugin names
in config_plugins table (such as 'auth/mnet') and the new correct
style (such as 'auth_mnet'). Attempting to rename the setting plugin via
low level SQL UPDATE could lead to duplicate key violation.

The patch introduces a new helper function to safely migrate the old
settings to the new ones, eventually informing the admin about the
values mismatch.