Automatic merge from submit-queue
Improve TerminationMessagePath to be more flexible
* Support `terminationMessagePolicy: fallbackToLogsOnError` which allows pod authors to get useful information from containers as per kubernetes/community#154
* Set an upper bound on the size of the termination message path or log output to prevent callers from DoSing the master
* Add tests for running as root, non-root, and for the new terminationMessagePolicy cases.
I set the limit to 4096 bytes, but this may be too high for large pod containers. Probably need to set an absolute bound, i.e. max message size allowed is 20k total, and we truncate if we're above that limit.
Fixes#31839, #23569
```release-note
A new field `terminationMessagePolicy` has been added to containers that allows a user to request `FallbackToLogsOnError`, which will read from the container's logs to populate the termination message if the user does not write to the termination message log file. The termination message file is now properly readable for end users and has a maximum size (4k bytes) to prevent abuse. Each pod may have up to 12k bytes of termination messages before the contents of each will be truncated.
```
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