Add FD_CLOEXEC option to all anonymous FDs

Upon execlp() some anonymous file descriptors from the kernel tracer
could be dup and thus making the refcount always > 1 on the lttng kernel
tracer side.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <david.goulet@polymtl.ca>