Fix: non-enabler events should be disabled by default

The session daemon expects that all events (enablers and events) are
disabled when created, so we can attach a filter before enabling them.

Also fix the kretprobe to ensure we can enable/disable both the entry
and return probe (--function instrumentation).

Fixes: #926
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>