osd/osdnet.cpp: Removed dependency from OSD network device to libemu

[cuavas]

This is required for #12054 to avoid fatal errors with network devices. This doesn’t completely clean up the interface to OSD network modules (it’s still using free functions and static variables in a really nasty way), but it does clean up a very nasty cross dependency.

There are still issues with the receive polling timer:

• The interval is set based on the MTU, assuming the rate packets arrive is no more than the theoretical maximum rate that maximal length packets can be serialised. However, smaller packets can arrive much faster, particularly when using jumbo frames (9000 octet MTU).
• That aside, the code was always based on the incorrect assumption that an emu_timer will synchronise with real time. It doesn’t, so for a system that’s fairly lightweight to emulate (e.g. an Apple II with a Uthernet card), it polls furiously while running the emulation for a frame, then doesn’t poll at all for the majority of the time while it waits for the real world to catch up before updating video and starting the next frame.
• It probably needs a postload handler to deal with the fact that changing the host network interface isn’t something that can be included in save states.

I realise this wreaks havoc with the macOS netdev PR. I haven’t forgotten. I’ll update that PR to deal with this myself.