ir: make bt_ctf_stream_create() public

ir: make bt_ctf_stream_create() public

This patch also removes bt_ctf_trace_create_stream(), so that the
only way to create a stream in non-writer mode is using
bt_ctf_stream_create(). Only the stream class is needed, and since
it needs to be part of a trace before calling the function, the
created stream gets associated to this trace on creation.

The function also knows if the stream class's trace was created by
a CTF writer thanks to a new flag. With this, it's able to create
a stream file and set the stream's FD if it's a writer stream.
Otherwise the FD is set to -1 and this stream cannot be passed to
stream writer functions like bt_ctf_stream_append_event() and
bt_ctf_stream_append_discarded_events(), for example.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>