Log containers which fail to stop during daemon shutdown

Followup to #6815

There are good reasons why we should not kill containers on a daemon shutdown - killing production-critical containers, for example. However, it's still not a good thing that daemon shutdown silently hangs if containers do not stop on a SIGTERM - this is potentially very confusing.

This patch causes containers which fail to stop after a SIGTERM to log this failure after a period of time, so it's clear why Docker is not stopping, and which containers would need to be stopped to permit a clean shutdown. Period of time is presently 1 minute, which is somewhat arbitrary - up for discussion on what a reasonable value is.