Count number of times a request to get an ethernet buffer could not be fulfilled

Currently there is no well-defined statistic available how often Driver Ethernet was unable to provide a buffer to prepare a frame.

In order to deliver this statistic, the already existing interface statistics of Driver Ethernet shall be extended. In particular, the field  ulOutErrors  is currently unused and shall be used to count this event. The counter shall be implemented directly in Driver Ethernet (and not its backends).