This is a JS implementation of the sbbs_t::privchat(), from chat.cpp.

It currently only supports local (sysop) chat (call it with the '-l' argument),
does not support split-screen chat (for the remote user), and needs a lot
more testing and optimization. But for now, it serves as an example of what
is needed to write a private/sysop-chat module in JS, compatible with the
SBBS-Win32 chat.exe, for example.