init: Fix HandleSignalFd()

There are two bugs in HandleSignalFd():
* If the one_off argument is true and if no data is read from the
  signalfd, 'siginfo' is left uninitialized and used in the switch
  statement in HandleSignalFd().
* The PLOG() statement in the switch statement should be a LOG()
  statement since it does not report a failed system call.

This CL has been tested by changing kDiagnosticTimeout locally from 10s
into 100ms.

Change-Id: I0e488dd95bc13e1befaef770c0748d1d47f6e431
Signed-off-by: Bart Van Assche <bvanassche@google.com>