init: Fix signalfd support in WaitToBeReaped()

Add an Epoll::Open() call such that the Epoll::Wait() calls wait for the
signalfd. Rename DiscardSiginfo() into HandleSignal(). Handle Epoll method
call failures. Test whether alive_pids is empty once per loop iteration.

Test: CtsInitTestCases:ServiceStopTest
Change-Id: I16c27aa74d61f889b304f3e59cd3c5255c299ce1
Signed-off-by: Bart Van Assche <bvanassche@google.com>