Fix DIZ extraction/use for FTP uploads

1. Was not setting f->dir to the correct directory number, so only ftp-uploads to the *first* directory (dirnum = 0) would extract DIZ files of uploaded files.

Removing the 'dirnum' parameter to addfile() since that implied that you did not have to initialize the 'dir' element of the passed file_t, but you do: to get the correct file path for file size/date detection and the DIZ extraction.

2. Was getting heap-corruption when freeing the imported/formatted DIZ text on Windows once the above problem was fixed: can't free() in one DLL memory that was allocated in another DLL. Created and now using free_diz() to free the memory allocated in read_diz().

format_diz() handles a NULL 'lines' argument correctly/gracefully, so no need for the NULL lines check in sbbs_t::uploadfile().

Added FTP server log messages for successful file upload or update by user.