Fix signal handler for handling all dead childs

Fix signal handler for handling all dead childs

The signal handler can be called once (not for each child)
when childs are terminated at very close timing each other.

reproduce : #pkgcmd --clear-all
	this causes all slots (rpm, tpk and wgt) active, and
	some processes are terminated at very close timing.

Change-Id: Ie007e17431e78e2920356e6178dff3af86826790
Signed-off-by: jongmyeongko <jongmyeong.ko@samsung.com>