D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/tools/diffutils/source/gnulib-tests/test-binary-io.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+tmpfiles=""
+trap 'rm -fr $tmpfiles' 1 2 3 15
+
+tmpfiles="$tmpfiles t-bin-out0.tmp t-bin-out1.tmp"
+./test-binary-io${EXEEXT} 1 > t-bin-out1.tmp || exit 1
+cmp t-bin-out0.tmp t-bin-out1.tmp > /dev/null || exit 1
+
+rm -fr $tmpfiles
+
+exit 0