core: add write support to current_control

By writing the current-control binary attribute, the user can
configure the device.

This patch introduce a new structure "zio_attr_config", it modifies
the prototype of the function t_op->config() and add a new zio device
function zio_device->config() for the device configuration.

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
tools/zio-control: a new GUI interface to read/write current_control

This interface allow you to inspect the current control of a channel.
You can change its attribute and the trigger's attributes.

At the moment, it is not possible to change the trigger type from this
interface.

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
UNTESTED zio-trigger.h: on output configure device before trigger fire

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
config.c: add control flags to ignore device or trigger configuration

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>