The now preferred way to call realpath() is by passing NULL as the
second argument and get a malloc'ed result. We still supported the
old way of providing our own buffer as a second argument, for some
platforms that didn't support the new way yet. Those were only
Solaris less than version 11 and some older AIX versions (7.1 and
newer appear to support the new variant). We don't support those
platforms versions anymore, so we can remove this extra code.
