core: Introduce accessor functions for structure private data

core: Introduce accessor functions for structure private data

The backend private data for the internal library structures has been
accessed through a zero-length os_priv array of type unsigned char.
This approach had two particular disadvantages:

  1) A special attribute was needed on the 'os_priv' member to ensure
     that the field was properly aligned to a natural pointer alignment.
     The support needed for this is not available in every compiler.

  2) Each access to the private data areas required an explicit cast
     from unsigned char to the type required by the backend.

This change reworks the way the private data is accessed by the
backends. New accessor functions return the private data as a void
pointer type, removing the need for an explicit cast (except for Haiku,
which is C++). The special alignment attribute trickery is also replaced
by simple pointer arithmetic.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>