Refactor the code that produces the name of the files returned by services

Refactor `suggest_dataset_basename` (which has fairly parallel implementations
in ssap and sia on top) to produce correct file names across platforms. Discuss whether for security reasons we'd want to limit the charset to ASCII printables. See related #557

Locate the parallel suggest_dataset_basename implementations in the ssap and sia modules, then read related issue #557 for context. Check how the generated names behave across platforms and resolve the open question about limiting characters to ASCII printables; done means both service paths produce correct, safe filenames.

Written by the indexing model from the issue text.