Werk #14079: Agent registration: Fix required user permissions

There were some user permission issues with the agent registration (cmk-agent-ctl register ...). For example, the registration crashed with "500 Internal Server Error" for users without the permission "Write access to all hosts and folders".

To register a host, users need the following permissions:

• Agent pairing.
• Read access to the host, either via "Read access to all hosts and folders" or via via contact groups.
• Write access to the host, either via "Write access to all hosts and folders" or via "Modify existing hosts" and write access to the specific host via contact groups.

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