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/ash_test/ash-misc/exitcode_trap5.right b/shell/ash_test/ash-misc/exitcode_trap5.right
new file mode 100644
index 0000000..1ad4443
--- /dev/null
+++ b/shell/ash_test/ash-misc/exitcode_trap5.right
@@ -0,0 +1,3 @@
+TERM
+Nested
+Zero:0