Make "BatchRenderItemList" comma safe

[derwin12]

If you batch render a sequence that contains commas, subsequent batch renders wont remember this value.
This is also an issue with the new fpp connect remembering the sequence.
config->Read("BatchRenderItemList", & itcsv, "");

Perhaps just clean the suggested file name that is currently based on the audio mp3.
Or perhaps it needs a better separator choice?