command is a special "hack" used by the drftpd server, but even though it is
a custom extension I've deemed it fine to add to libcurl since this server
seems to survive and people keep using it and want libcurl to support
it. The new libcurl option is named CURLOPT_FTP_USE_PRET, and it is also
usable from the curl tool with --ftp-pret. Using this option on a server
that doesn't support this command will make libcurl fail.
