csource: fix parallel mode to wait for subprocesses
csource: fix parallel mode to wait for subprocesses

Currently it lefts some orphaned children,
so that ctrl+C does not kill them.
Wait for the children.

hyz0906/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,xairy/syzkaller,zoulasc/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,dotweiba/syzkaller,hyz0906/syzkaller,hyz0906/syzkaller,dotweiba/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,zoulasc/syzkaller,xairy/syzkaller,hyz0906/syzkaller,dotweiba/syzkaller,hyz0906/syzkaller,google/syzkaller,xairy/syzkaller,zoulasc/syzkaller,google/syzkaller,google/syzkaller