hush: fix "wait PID"

hush: fix "wait PID"

It was not properly interruptible, and did not update job status
(the exited processes were still thought of as running).

function                                             old     new   delta
process_wait_result                                    -     453    +453
wait_for_child_or_signal                               -     199    +199
run_list                                             996    1002      +6
checkjobs_and_fg_shell                                41      43      +2
builtin_wait                                         328     215    -113
checkjobs                                            516     142    -374
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 2/2 up/down: 660/-487)          Total: 173 bytes

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