Fix how command line handles Insert/Overwrite mode.

[TomAwezome]

If the Insert key is pressed at a command line prompt, it enters a mode where newly written text will overwrite text to the right rather than shift it over. This functionality works well in Ed windows and forms, but on the command line pressing Enter to execute a command while in Insert mode will send the cursor from wherever it is to the top of the window, along with most likely not successfully executing the command.