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/tests/binary

@@ -0,0 +1,15 @@
+#!/bin/sh
+# small examples
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+printf 'Binary files - and /dev/null differ\n' > out-exp || fail_ setup
+
+fail=0
+
+# diff must exit with status 1, stdout as above, and no stderr.
+printf '\0'| returns_ 1 diff - /dev/null > out 2> err || fail=1
+compare out-exp out || fail=1
+compare /dev/null err || fail=1
+
+Exit $fail