stream: rearrange open functions

Add yet another variant of the stream open function. This time, make it
so that it's possible to add new open parameters in an extendable way,
which should put an end to having to change this every other year.

Effectively get rid of the overly special stream_create_instance()
function and use the new one instead, which requires changes in
stream_concat.c and stream_memory.c. The function is still in private in
stream.c, but I preferred to make the mentioned users go through the new
function for orthogonality. The error handling (mostly logging) was
adjusted accordingly.

This should not have any functional changes. (To preempt any excuses, I
didn't actually test stream_concat and stream_memory.)