C-style memory management is a pain and nearly nobody seems to get
it right. Add a C++-version of membuffer that frees the buffer
when it gets out-of-scope. Originally, I was thinking about
conditionally adding a constructor/destructor pair when compiling
with C++. But then decided to create a derived class membufferpp,
because it would be extremely confusing to have behavioral change
when changing a source from from C to C++ or vice-versa.
Also add a comment about the dangers of returned pointer: They
become dangling on changes to the membuffer.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
