Add a ton of debugging statements together with "debug" configuration setting

It turned out that one simply needs debugging statements when things don't work as expected.
Before everyone has to add these himself, we add them once and for all, accepting the cost of
blowing up the source by a fair amount. Simply enable the "debug" option in the plugin's
configuration.
Fix createUser() for existing users in primary auth plugin that can create users

We always tried to create a user if the primary auth plugin supports creating users. This
naturally failed if the user already existed. It didn't show up with authhttp as primary
auth plugin as it doesn't support adding users, whereas automysql does.

Reported by ssahara on Github.