MEGA65: ethernet emulation update and fixes (tap interface)

[lgblgblgb]

Xemu/MEGA65 has ethernet emulation (using TAP driver in Linux, so it's kinda Linux specific) however:

1. It emulates an older behaviour of MEGA65 which has changed since then, and does not work this way any more
2. It had bugs, since it was not thread-safe
3. No IRQ was emulated
4. Since it uses Linux TAP driver, it's Linux specific (it's possible that some other UNIX-like system works, even MacOS, but surely not Windows, for example)
5. It would be nice to have a "fake" DHCP server in Xemu, so if a MEGA65 software uses DHCP to get its IP, it will get one. it would greatly decrease the network setup complexity needed for Xemu without the requirement of a real DHCP server bound to the tap interface which Xemu can use

This issue is intended to remedy at least point 1 from the list above, ideally point 2 as well. For point three, a totally different approach should be used (or "as well"), what I have no idea about, yet (what can use on Windows for similar purpose, if it's possible at all).

Currently mega65-core has some issues with ethernet: MEGA65/mega65-core#523 It's wise to see it being "perfect" then trying to refactor my ethernet emulation to match that (so it can be compared) with the changes.