Redmine#4096: data_readstringarray and data_readstringarrayidx functions

See https://cfengine.com/dev/issues/4096

Adds much, much simpler `data_readstringarray` and `data_readstringarrayidx` functions, compared to their predecessors `readstringarray` and `readstringarrayidx`.  They work exactly the same way but return a data container instead of installing one variable for each array entry(!).

Acceptance test included, will add more test cases if requested.

Documentation to follow upon merge.