* On output: detect them and transcode them properly into UTF-8.
* On input: create input records for both halfs of the surrogate pair.
I tested by copying-and-pasting U+20000 (D840 DC00) into a mintty bash
shell. It works, but the Windows console thinks the character occupies
four cells when it really occupies two, so the cursor position is wrong.
When I press backspace, it doesn't delete the whole character -- instead,
it replaces it with a '?', because it becomes an invalid surrogate pair.
Still, the behavior seems like an improvement.
