Always choose large event header for UST channels

UST can receive the session start command before all probe provider
library constructors have completed running, therefore finding less
events than eventually enabled within the process. Moreover, with
per-uid buffers, many processes end up registering events into shared
buffers. Therefore, the guess based on number of events from the first
process to use the buffer is incorrect.

Considering that we typically have applications with more than 30
events, we will modify the session daemon so it selects the "large"
header type independently of the number of events.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>