tty: buffer incomplete command from previous syscall

After changes in this MR, sending an empty newline evaluates in lua
as nil, and that seems fine.  Let's drop this piece of code;
it was broken now anyway (incorrect `io_mode_text` part).

No other change, for clarity.

These are not TCP connections.  It has worked fine so far, but...

We don't need them anywhere, so let's keep their scope small.

Lukas Jezek authored 4 years ago and Vladimír Čunát committed 4 years ago

vcunat change some parts of this, mainly around memory allocation.