Avoid conflicts with similar declaration of MAX_SOCK_NUM

[Max19thl]

Great! it was surprise for me now, that yield() also defined as weak function. Thanks!
However, definition of some ethernet library specific handler might allow to avoid re-enterability issues when ethernet functions will be invoked indirectly from yield() handler
in case if some "ethernetYield" will be redefined in sketch , it will be possible to raise some global flags before normal yield() to prevent nested lib invocation.
So, my suggestion to use ethernetYield() weak handler execution in loops inside lib, and to define default weak handler with normal yield(); execution inside
It looks slightly more flexible.

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