Generate local kernel and UST indexes

When the consumer creates a new trace file, it now creates as well an
index file that the viewers can use to navigate in the trace.
For now these indexes are only local and not sent to the relayd.

This patch requires additionnal ioctl and ustctl calls implemented
in lttng-modules and lttng-ust in 2.4.

For now, no indexes are sent over the network and no indexes are
generated on snaphots.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>