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/signalblocking.m4

@@ -0,0 +1,25 @@
+# signalblocking.m4 serial 14
+dnl Copyright (C) 2001-2002, 2006-2017 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.
+
+# Determine available signal blocking primitives. Three different APIs exist:
+# 1) POSIX: sigemptyset, sigaddset, sigprocmask
+# 2) SYSV: sighold, sigrelse
+# 3) BSD: sigblock, sigsetmask
+# For simplicity, here we check only for the POSIX signal blocking.
+AC_DEFUN([gl_SIGNALBLOCKING],
+[
+  AC_REQUIRE([gl_SIGNAL_H_DEFAULTS])
+  AC_REQUIRE([gl_CHECK_TYPE_SIGSET_T])
+  if test $gl_cv_type_sigset_t = yes; then
+    AC_CHECK_FUNC([sigprocmask], [gl_cv_func_sigprocmask=1])
+  fi
+  if test -z "$gl_cv_func_sigprocmask"; then
+    HAVE_POSIX_SIGNALBLOCKING=0
+  fi
+])
+
+# Prerequisites of lib/sigprocmask.c.
+AC_DEFUN([gl_PREREQ_SIGPROCMASK], [:])