Fix a couple of I/O bugs

[zvea]

Fixed two bugs:

• off-by-one error when appending newlines
• recv() was assumed to always return complete messages, whereas in fact it can return partial messages (the connection uses a stream socket)

This significantly improves compatibility with https://github.com/Doridian/carvera-pendant

However, some compatibility issues remain. Clout frequently misses pendant steps, whereas the first-party controller works flawlessly. This suggest a controller, rather than a pendant, issue. I'll keep playing with it and fixing things I find.