“Support” X·M·L 1.1

“Support” X·M·L 1.1

The X·M·L 1.1 “support” amounts to deleting the declaration and
replacing any character escapes for C·0 controls with
`U+0091 PRIVATE USE ONE`, which is a valid character in X·M·L 1.0.
This is done entirely in `sed`, so it’s not perfect, but it should be
“good enough”.