Introduce lazy system call event creation

Introduce lazy system call event creation. If only specific system call
events are enabled, only create the metadata for those.

Align implementation of system call event probes for recording events
with their notifier counterpart.

This is part of the background work required for a future feature (trace
hit counters and map support) which brings benefits on its own.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>