method would insert keys at end of the input ring buffer. This was fine if
there weren't already chars in the buffer. But if there were already chars in
the input buffer, ansi_getxy() would then move keys to the end of the input
buffer until it found the <ESC>[y;xR response it was looking for. This garbled
the user's input in split-screen chat as reported by JasHud and others.
If there was a RingBufInsert() routine, we could use that instead, but this
separate "ungetkey" buf will do for now.