Un-constify getProgramName return value

[danra]

getProgramName unconventionally returns const String rather than just String, and so triggers various warnings. Besides having no advantage AFAICS*, clang-tidy triggers the readability-const-return-type warning on this.

This is a breaking change for existing overriders, though (*which is a valid reason to keep as-is, though not a great one), so if the pull request is accepted it should also be listed under the breaking changes.