Werk #14732: cmk-update-agent: Retry locking

The agent updater is designed to have no more than one instance running at a time on a host. This is implemented by holding a file lock while running.

We recently observed that a lock may sometimes fail briefly after it was released by a previous agent updater instance, possibly due to some anti virus software.

To mitigate this situation, the locking is now retried 10 times before aborting the agent updater call.

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