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/bignum

@@ -0,0 +1,14 @@
+#!/bin/sh
+# big numbers
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+for tabsize in 2147483648 9223372036854775808; do
+  diff --tabsize=$tabsize /dev/null /dev/null
+  status=$?
+  test $status -eq 0 || test $status -eq 2 || fail=1
+done
+
+Exit $fail