improve TCP state mechanics support

For the most part, this patch adds graceful TCP connection shutdown behaviour to class TCPConnection in fake_network.js.

Network-intense guest applications like "apt" run noticeably smoother with support for graceful TCP shutdown.

Changes in detail:

- completed SYN-related state mechanics (TCP connection establishment)
- added FIN-related state mechanics (TCP connection shutdown)
- reordered and regrouped TCP packet handling steps in TCPConnection.process()
- reduced minimum IPv4 packet size from 46 to 40 bytes to reduce noise in logs (why is this even checked)
- added explicit detection of TCP Keep-Alive packets to reduce noise in logs
- added dbg_log() calls to trace TCP connection state (could be removed if unwanted, or maybe use smth. like LOG_TCP instead of LOG_FETCH?)
- removed TCPConnection.seq_history as it was not used anywhere

As should be expected, the changes related to TCP connection state only affect class TCPConnection internally and are mostly concentrated in its methods process() and pump().

A few TCP connection states are left unimplemented, reasons:

- CLOSING: simultaneous close from both ends (deemed too unlikely)
- TIME_WAIT: wait seconds to minutes after sending final ACK before entering CLOSED state (pointless in our specific case and would require a Timer)
- LISTEN: are server sockets even supported? Both WISP and FETCH do not support them. Also, TCPConnection.connect() seems not to be called from anywhere (it used to be called from fake_tcp_connect() which was deleted in the previous patcH).

As before, fake_network.js does not use any Timers (i.e. setTimeout()), the code is purely driven by outside callers. That means that there are no checks for lost packets (or resends), the internal network is assumed to be ideal (as was the case before).

Even though I fixed and removed most of the TODOs I originally found in fake_network.js, I've added several new ones that are left to be discussed.

Tested with V86 network providers WispNetworkAdapter and FetchNetworkAdapter, and with V86 network devices ne2k (FreeDOS) and virtio (Debian 12 32-bit).