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/m4/sockets.m4

@@ -0,0 +1,17 @@
+# sockets.m4 serial 7
+dnl Copyright (C) 2008-2017 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_SOCKETS],
+[
+  AC_REQUIRE([AC_C_INLINE])
+  AC_REQUIRE([gl_SOCKETLIB])
+  gl_PREREQ_SOCKETS
+])
+
+# Prerequisites of lib/sockets.c.
+AC_DEFUN([gl_PREREQ_SOCKETS], [
+  :
+])