Using different defaults about logging for vs/pure-ftpd (bnc#888287)

Also switching from "syslog_enable" to  "xferlog_enable", which makes more sense.
This is vsftpd, so it always uses `upcase`, what about changing it to ... ?

    return (@VS_SETTINGS["log_ftp_protocol"] || @DEFAULT_CONFIG["log_ftp_protocol"]).upcase