Introduce LTTNG_KERNEL_SESSION_SET_NAME

The tracer controller (lttng-sessiond) can now inform the kernel tracer
of the name of the created session. This will allow the tracer to
propagate the information inside the trace metadata under a "env" field.

This information is normally inferred from the generated folder
structure where a trace rests.

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>