iio-sensors: Add infrastructure for IIO events

iio-sensors: Add infrastructure for IIO events

This introduces event_descriptor_t to represent an event.
Each channel can have multiple events attached.

Activating a sensor of type EVENT implies:
	* retrieving associated event_fd file descriptor (see. IIO_GET_EVENT_FD_IOCTL)
	* adding event_fd to the watched fds poll set
	* enabling IIO event, usually writing 1 to iio:deviceX/events/in_<event_name>_en

Deactivation does the above operations in reverse order.

Change-Id: I305f28456b43ea8a662e49c2c3aa8febd0d7d84f
Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>