Add a website FTP access user option unavailable on GPL

For some reason I thought this change had already been checked into 3.50, but I see that it is still wrapped in a Pro check.

I can't think of any reason for this to be a Pro only feature.

There's a thread about it in the forums:

http://www.virtualmin.com/forums/limit/10/limitstart/10/virtualmin/re%3a...