Add basic termios support

[ianthomas23]

Add basic termios support. This is needed for interactive commands such as vim.

It overrides the emscripten functions TTY.default_tty_ops.ioctl_tcgets and tcsets to store and use a local version of the termios structure defined in termios.ts. Initially it only supports use of the output flag ONLCR to control conversion of \n to \r\n, more will be added as needed.

There is a new wasm command check_termios included, both source and compiled code, so that termios settings can be tested. You pass in the flags you want to chance as command arguments, then it loops accepting stdin and writing to stdout.

This is inspired by the marvellous xterm-pty.