fix anonymous mode

[robklg]

unFTP currently runs either with an authenticator that implements actual user authentication, and anonymous access. But in various places in the code, we match on user names 'anonymous' or 'ftp'. While anonymous authenticator accepts any random username as valid...

So the fix should either be to let the anonymous authenticator allow only the 'anonymous' or 'ftp' account names, or create a function somewhere that can be called to figure out if unFTP is in anon mode or not.