This adds a new "arrow" key "delete" corresponding to the kD termcap value.
It only works if that is a sequence such as "\033[3~"; if it is "\177", the
em-delete-prev-char or ed-delete-prev-char from the single-character
mappings remains. It turns out that most terminals (xterm and alikes,
syscons in xterm mode) produce "\033[3~" by default so <Delete> has the
expected effect.
This also means that things need to be considerably misconfigured for
<Backspace> to perform a <Delete> action.
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