lib: work around talloc_steal usage from C++ code

lib: work around talloc_steal usage from C++ code

Implicit typecast from 'void *' to 'T *' is okay in C, but not in
C++. In talloc_steal, an explicit cast is provided for type safety in
some GCC versions. Otherwise, a cast is required. Provide a template
function for this to maintain type safety, and redefine talloc_steal
to use it.

The template must be outside the extern "C" block (NOTMUCH_BEGIN_DECLS
and NOTMUCH_END_DECLS), but keep it within the GCC visibility #pragma.

No functional changes, apart from making the library build with
compilers other than recent GCC.

Signed-off-by: Jani Nikula <jani@nikula.org>