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-sys_uio.c

@@ -0,0 +1,32 @@
+/* Test of <sys/uio.h> substitute.
+   Copyright (C) 2011-2017 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Eric Blake <eblake@redhat.com>, 2011.  */
+
+#include <config.h>
+
+#include <sys/uio.h>
+
+/* Check that necessary types are defined.  */
+size_t a;
+ssize_t b;
+struct iovec c;
+
+int
+main (void)
+{
+  return a + b + !!c.iov_base + c.iov_len;
+}