AqBanking6: More work on porting backends and tools. Added AB_Provider_Control().
- AB_Provider_Control() is a variable extension provided by backends which
can be used by e.g. setup tools
- with this mechanism in place we no longer need to provide backend
libraries, instead the backend-specific setup tools (like aqhbci-tool)
just link against libaqbanking and use the control() mechanism to access
- those control functions can already be called from the tool aqbanking-cli
"aqbanking-cli --control=aqhbci mkpinlist"