Add emulated serial port

[giomba]

Desciption

This pull request implements an emulated serial port which you can use to communicate with the emulated software. Serial port is emulated as a TCP socket.

Review by commit is possible, but full-review is okay too.

Usage

1. type serial open in the emulator's command line (with an optional port number): the emulator will start listening for an incoming connection at the specified port (or 52955 (0xCEDB) in case no port is given)
2. you can use socat to emulate a serial port which can be used by serial port libraries, and connect it to the TCP socket, for example: socat -v -d -d pty TCP4:127.0.0.1:52955 (the -d -d option is used to show which virtual /dev/pts is created)
3. connect to the given /dev/pts/x using your favorite application/serial terminal emulator/kermit client/whatever

[giuliof]

Comment "en passant": may be a good idea to use, if I recall their names, a "named pipe" instead of TCP socket. In this way the serial port is actually mapped to a file and can be accessed like a real serial port, avoiding socat.
Named pipes and TCP sockets are interchangeable and the usage of the former or the latter can be switched through config file, CLI, or - better- program option.

[luigix25]

IIUC you never check for return value

[giomba]

This module implements the CEDAModule interface (see src/module.h), which means that this function is polled in the main loop, so the check is performed during next loop iteration, and (if I did not get something wrong) nothing will break.
Anyhow, I'm always printing the "accept client" message, which may actually be wrong. Fixed.

[luigix25]

nit: why not squashing this commit in the previous one?

[giomba]

I kept two different commits because the first one (Implement TX-ready channel flag in SIO/2) actually implements a new feature, which can be kept in its independent commit, while this one (Remove TODO comments for virtual serial port emulation) is more like a "chore" commit where I just remove some outdated comments (some of which are related to the implementation in the previous commit, but others aren't, like the ones referring to the keyboard).

[giomba]

@giuliof

may be a good idea to use, if I recall their names, a "named pipe" instead of TCP socket

I tought about using a pipe, but decided to go with the TCP solution, because I've already seen and used it on other emulators (eg. Vice for the Commodore machines).

Named pipes and TCP sockets are interchangeable and the usage of the former or the latter can be switched through config file, CLI, or - better- program option.

You can open a follow-up issue.