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-c-stack.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+tmpfiles=""
+trap 'rm -fr $tmpfiles' 1 2 3 15
+
+tmpfiles="t-c-stack.tmp"
+./test-c-stack${EXEEXT} 2> t-c-stack.tmp
+case $? in
+  77) cat t-c-stack.tmp >&2; (exit 77); exit 77 ;;
+  1) ;;
+  *) (exit 1); exit 1 ;;
+esac
+if grep 'stack overflow' t-c-stack.tmp >/dev/null ; then
+  :
+else
+  (exit 1); exit 1
+fi
+
+rm -fr $tmpfiles
+
+exit 0