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-perror.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+: ${srcdir=.}
+. "$srcdir/init.sh"; path_prepend_ .
+
+# Test NULL prefix. Result should not contain a number, except in lines that
+# start with 'EDC' (IBM z/OS libc produces an error identifier before the
+# error message).
+test-perror 2>&1 >/dev/null | LC_ALL=C tr -d '\r' > t-perror.tmp
+grep -v '^EDC' t-perror.tmp | grep '[0-9]' > /dev/null \
+  && fail_ "result should not contain a number"
+
+# Test empty prefix. Result should be the same.
+test-perror '' 2>&1 >/dev/null | LC_ALL=C tr -d '\r' > t-perror1.tmp
+diff t-perror.tmp t-perror1.tmp \
+  || fail_ "empty prefix should behave like NULL argument"
+
+# Test non-empty prefix.
+test-perror foo 2>&1 >/dev/null | LC_ALL=C tr -d '\r' > t-perror3.tmp
+sed -e 's/^/foo: /' < t-perror.tmp > t-perror2.tmp
+diff t-perror2.tmp t-perror3.tmp || fail_ "prefix applied incorrectly"
+
+# Test exit status.
+test-perror >out 2>/dev/null || fail_ "unexpected exit status"
+test -s out && fail_ "unexpected output"
+
+Exit 0