Standardize on "metadata" as the description of a file's "tail" dfield

It's anticipated that this will be used for JS-populated file metadata in JSON format in the future (and not just "archive contents" in .ini format).

Also, fix the double-free issue that was occurring when moving files with extended file descriptions (sbbs_t::movefile()). This was actually the primary problem I was fixing here, but noticed the metadata issue: metadata would not have been moved along with the other file info between bases.