init: Log more information if stopping times out

The newly logged information includes the pending signals bitmask (SigPnd).
That information allows to determine whether kill() did not deliver a
signal or whether SIGCHLD has not been processed in time. This patch is
expected to help with root-causing b/254878413.

Bug: 254878413
Change-Id: Id33d1fb724163f8e5d4b3edfeb6c4be5321c0d95
Signed-off-by: Bart Van Assche <bvanassche@google.com>