Local backend: Enable/disable buffer mode when opening/closing device
Local backend: Enable/disable buffer mode when opening/closing device

Also, return -EBUSY instead of -EAGAIN if the dev node has already
been opened.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

Sunderfield/libiio,gburca/libiio,gburca/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,Sunderfield/libiio,gburca/libiio,Sunderfield/libiio,sensarliar/libiio,analogdevicesinc/libiio,sensarliar/libiio,sensarliar/libiio,analogdevicesinc/libiio