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/tests/label-vs-func

@@ -0,0 +1,31 @@
+#!/bin/sh
+# Show how diff's -p option can mistakenly match a label: in column 1.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+cat <<EOF > exp || fail=1
+@@ -4 +4 @@ label:
+-1;
++2;
+EOF
+
+cat <<EOF > a || fail=1
+func ()
+{
+label:
+1;
+}
+EOF
+
+sed s/1/2/ a > b || fail=1
+returns_ 1 diff -p -u0 a b > out 2> err || fail=1
+
+tail -3 out > k && mv k out || fail=1
+
+compare exp out || fail=1
+
+# expect empty stderr
+compare /dev/null err || fail=1
+
+Exit $fail