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-verify.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+. "${srcdir=.}/init.sh"
+
+# We are not interested in triggering bugs in the compilers and tools
+# (such as gcc 4.3.1 on openSUSE 11.0).
+unset MALLOC_PERTURB_
+
+# Rather than figure out how to invoke the compiler with the right
+# include path ourselves, we let make do it:
+(cd "$initial_cwd_" \
+ && rm -f test-verify-try.o \
+ && $MAKE test-verify-try.o >/dev/null 2>&1) \
+  || skip_ "cannot compile error-free"
+
+# Now, prove that we encounter all expected compilation failures:
+: >out
+: >err
+for i in 1 2 3 4 5; do
+  (cd "$initial_cwd_"
+   rm -f test-verify-try.o
+   $MAKE CFLAGS=-DEXP_FAIL=$i test-verify-try.o) >>out 2>>err \
+  && { warn_ "compiler didn't detect verification failure $i"; fail=1; }
+done
+
+Exit $fail