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-mbsinit.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# Test whether a specific UTF-8 locale is installed.
+: ${LOCALE_FR_UTF8=fr_FR.UTF-8}
+if test $LOCALE_FR_UTF8 = none; then
+  if test -f /usr/bin/localedef; then
+    echo "Skipping test: no french Unicode locale is installed"
+  else
+    echo "Skipping test: no french Unicode locale is supported"
+  fi
+  exit 77
+fi
+
+LC_ALL=$LOCALE_FR_UTF8 \
+./test-mbsinit${EXEEXT}