Fix uploader-notification, credit awards, download-counters in FTP downloads

Since v3.19 (the new filebases), when a user FTP-downloaded a file, we failed
to properly find/load that file's record from the filebase (searching for the
file's full path, rather than just the filename), so the code the increments
the file's download counter, notifies the uploader, awards credits, etc. did
not ever execute. This means that FTP-downloads for all files downloaded via
FTP were effectively "free" (and nobody noticed). No error was logged either.

I discovered this while debugging the case of "(null)" filenames in the
action/download MQTT topic messages being published by the FTP server. So
that issue is fixed as part of this commit as well.

Oh, and if this code had executed before, it would have memory-leaked the
file information, so that's fixed too (added call to smb_freefilemem). Ugh.