WL#1624: determine MAC addr on Windows

UUID() (WL#1325) uses hardware address of an interface (ethernet addr) - if
there are many, any one can be used.

On Windows MAC addr detection does not work - it has to be fixed.

An example of how to find mac addr on Windows one can see in libnet.

As relevant win32 file is under new BSD license (w/o advertising clause)
we can simply reuse the code, copying it into mysys/my_gethwaddr.c