busybox

hush: make "wait %1" less likely to play with signal mask

Was playing with "sleep 3 | exit 3 & wait %1" and noticed that often
SIGCHLD arrives even before I get to signal masking. Can avoid it in this
case.

function                                             old     new   delta
wait_for_child_or_signal                             228     265     +37

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