Extract adding a trace to the iterator from bt_iter_init

With live tracing, we have cases where we need to add traces to an
iterator currently in use. Here, we just extract the code from
bt_iter_init that adds the trace to the iterator so we can call it from
elsewhere.
We don't necessarily want to expose this interface to the public, so it
stays in the internal headers for now.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Rename lttng-viewer.h to lttng-viewer-abi.h

Match lttng-tools filename.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Sync lttng live ABI with lttng-tools

Just add the structures for two new commands.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Create the live viewer session before attaching

The lttng-live protocol now requires that we create a viewer session
before attaching to a trace.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
get_new_streams and multi-session

We now support the get_new_stream lttng live command. This command
allows us to retrieve new streams added while tracing.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>