lttng: debugfs and procfs ABI

Add the "lttng" virtual file to debugfs and procfs. All operations are
performed through ioctls (LTTng ioctl range is already reserved
upstream) on this virtual file and on anonymous file descriptors
returned by these ioctls. Each file descriptor is associated with a
tracer "object" (session, channel, stream, event, context).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>