Do waitpid() for ALL children, even if there are no threads waiting

Do waitpid() for ALL children, even if there are no threads waiting

i.e. call radius_exec_program() 4 times for each packet, each
with "nowait".  The "reap_children" function is called ONCE per packet,
and was calling waitpid() ONCE if there were no threads waiting
for a child.

This change makes it KEEP waiting until there are no more children
to wait for.