hush: implement "return NUM in trap sets $? after trap"

function                                             old     new   delta
builtin_return                                        47      67     +20
check_and_run_traps                                  243     259     +16
run_pipe                                            1583    1597     +14
hush_main                                           1076    1086     +10
run_list                                            1054    1055      +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/0 up/down: 61/0)               Total: 61 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-misc/exitcode_trap5.tests b/shell/hush_test/hush-misc/exitcode_trap5.tests
new file mode 100755
index 0000000..332a064
--- /dev/null
+++ b/shell/hush_test/hush-misc/exitcode_trap5.tests
@@ -0,0 +1,10 @@
+# "return" in trap sets $? after trap...
+# ...but not a nested one!
+g() { echo Nested; return 22; }
+trap "echo TERM;false;g" term
+f() {
+	(kill $$) &
+	sleep 1
+}
+f
+echo Zero:$?