Some naming cleanup of the filename prefix blacklist added in r25620:

Some naming cleanup of the filename prefix blacklist added in r25620:
* Dropped a redundant if(count()) around a foreach().
* Renamed SpecialUpload::getFilenamePrefix() to getFilenamePrefixBlacklist(), since it returns a blacklist of filename prefixes, not the prefix of a filename. :)
* Renamed some local variables to match
* Renamed 'filename-prefix' message to 'filename-bad-prefix' and 'filename-prefix-list' to 'filename-prefix-blacklist' to clarify usage.