hush: make "exit" in trap use pre-trap exitcode

hush: make "exit" in trap use pre-trap exitcode

function                                             old     new   delta
check_and_run_traps                                  259     276     +17
builtin_exit                                          42      53     +11
hush_main                                           1086    1096     +10
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(add/remove: 0/0 grow/shrink: 3/0 up/down: 38/0)               Total: 38 bytes

Fixes exitcode_trap2.tests.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>