The public header files are moved to a new subdirectory, to separate
the definition of the public interface from the actual implementation.
Using an identical directory layout as the final installation has the
advantage that the example code can be build outside the project tree
without any modifications to the #include statements.
Using a resizable memory buffer allows to allocate the right amount of
memory inside the backend, avoiding having to know the required buffer
size in advance.
When necessary, an output parameter is added to provide the size
information. Status codes are strictly reserved for providing status
information only.
