As a result of fixing the auth plugins config storage in MDL-12689, many
settings would be falsely reported as new ones by
admin/upgradesettings.php. We do not want to confuse admins so we try to
reduce the bewilderment by pre-populating the config_plugins table with
default values. This should be done only for disabled auth methods. The
enabled methods have their settings already stored, so reporting actual
new settings for them is valid.
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.
