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-stack2.sh

@@ -0,0 +1,36 @@
+#!/bin/sh
+
+tmpfiles=""
+trap 'rm -fr $tmpfiles' 1 2 3 15
+
+tmpfiles="t-c-stack2.tmp"
+
+# Sanitize exit status within a subshell, since some shells fail to
+# redirect stderr on their message about death due to signal.
+(./test-c-stack${EXEEXT} 1; exit $?) 2> t-c-stack2.tmp
+
+case $? in
+  77) if grep 'stack overflow' t-c-stack2.tmp >/dev/null ; then
+        if test -z "$LIBSIGSEGV"; then
+          echo 'cannot tell stack overflow from crash; consider installing libsigsegv' >&2
+          exit 77
+        else
+          echo 'cannot tell stack overflow from crash, in spite of libsigsegv' >&2
+          exit 1
+        fi
+      else
+        cat t-c-stack2.tmp >&2
+        exit 77
+      fi
+      ;;
+  0) (exit 1); exit 1 ;;
+esac
+if grep 'program error' t-c-stack2.tmp >/dev/null ; then
+  :
+else
+  (exit 1); exit 1
+fi
+
+rm -fr $tmpfiles
+
+exit 0