Make using C++ type with snew() a compiler error

Make using C++ type with snew() a compiler error

Make it impossible to use snew() and friends on a C++ type (now gives a
compiler error).  This is possible with C++11, and makes it much less
error-prone to start introducing C++ code into existing C structs.

As a side effect, this enforces also proper encapsulation of memory
management for opaque structs.  Fixed a few instances where this wasn't
the case.

CUDA code skips these assertions.

Change-Id: I470dc077bb82c6ca7da5fb9a2daa64211b6d632c