libafb-helpers: Provide a way to get binding directory

Here is an example of application framework binder launch to show the difference between 'binder root directory' and 'binding directory' :
`afb-daemon --name=afb-example --rootdir=/home/example/binder_rootdir --ldpaths=/home/example/other_bindings_dir --binding=/home/example/test_binding_dir/test.so -vvv`
In that configuration the 'binder root directory' is '/home/example/binder_rootdir', and it's the case for all bindings launched within the binder.
Nevertheless, for the binding 'test.so', the 'binding directory' is '/home/example/test_binding_dir/'. Moreover, if other bindings are found using '--ldpath=$PATH_TO_BINDINGS' option, all these bindings will have different 'binding directory' depending where they are located.

Current 'GetBindingDirPath' function is giving 'binder root directory' path. That's why I wanted to rename current 'GetBindingDirPath' to 'GetBinderRootDirPath'. And to implement a new 'GetBindingDirPath' function to be able to get 'binding directory'.

For regular afb service, the 'binder root directory' is set within the systemd service file. Often, an afb service binding is located in '$PATH_TO_AFB_SERVICE/lib/$NAME_OF_THE_BINDING.so', so the 'GetBindingDirPath' I want to implement would return '$PATH_TO_AFB_SERVICE/lib' where the current 'GetBindingDirPath' returns '$PATH_TO_AFB_SERVICE'.

The correction is required to improve 'libappcontroller' way of finding its plugins, especially when multiple bindings are launched within the same binder.