...which only happens ot work in environments where sizeof (bool) == 1.  The
simpler alternative is to use the operator <<= template without passing explicit
UNO type information, anyway.

The std::nullptr_t overloads are needed to disambiguate calls with a nullptr
argument.  (Which can at least be meaningful for VOID, but for other types what
it happens to do is store a default value of the given type.)  As std::nullptr_t
is only C++11, this all needs to be LIBO_INTERNAL_ONLY.

Change-Id: Iff06a6ba94250bd4ae4afc937c2a2bfa75f0888f