Added API function iio_context_set_timeout

This function can be used to change the default timeout for I/O operations
(iio_buffer_refill, iio_buffer_push etc.).

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
IIOD: Added a TIMEOUT command, to set/get the timeout for I/O ops

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Local backend: Added implementation of function local_set_timeout

The default timeout is set to one second.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Network backend: Added implementation of function network_set_timeout

The default timeout is set to five seconds.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>