Creating a new output stream of a given type is currently done by
calling new_<type>_stream(), which all start by calling
new_output_stream() to allocate the stream and do common init, followed
by type-specific init.
Reverse this structure - the caller now calls the common function
ost_add() with the type as a parameter, which then calls the
type-specific function internally. This will allow adding common code
that runs after type-specific code in future commits.
9bae55165d