Werk #8674: Do not require a "hostname|" prefix when querying services columns in the host table.

While the requirement makes sense for the servicegroups table (there can be multiple hosts with the same service), it is actually harmful when querying the host table: One can't formulate queries like "give me all hosts which don't have service FOO".

To keep backwards compatibility and enabling such queries, the "hostname|" prefix is optional for those columns in the host table now.

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