sed: support GNU-like '\t' escape in substitutions

testsuite/README

@@ -4,7 +4,9 @@ Update: doesn't work as described. Try "make check" from parent dir...
 To run the test suite, change to this directory and run "./runtest".  It will
 run all of the test cases, and list those with unexpected outcomes.  Adding the
 -v option will cause it to show expected outcomes as well.  To only run the test
-cases for particular applets, specify them as parameters to runtest.
+cases for particular applets:
+
+./runtest <applet1> <applet2>...
 
 The test cases for an applet reside in the subdirectory of the applet name.  The
 name of the test case should be the assertion that is tested.  The test case

testsuite/sed.tests

@@ -57,6 +57,7 @@ testing "sed s arbitrary delimiter" "sed -e 's woo boing '" "boing\n" "" "woo\n"
 testing "sed s chains" "sed -e s/foo/bar/ -e s/bar/baz/" "baz\n" "" "foo\n"
 testing "sed s chains2" "sed -e s/foo/bar/ -e s/baz/nee/" "bar\n" "" "foo\n"
 testing "sed s [delimiter]" "sed -e 's@[@]@@'" "onetwo" "" "one@two"
+testing "sed s with \\t (GNU ext)" "sed 's/\t/ /'" "one two" "" "one\ttwo"
 
 # branch
 testing "sed b (branch)" "sed -e 'b one;p;: one'" "foo\n" "" "foo\n"