Introduced FreeDownloadUserMsg text.dat string

Tired of being reminded that you were "awarded 0 credits"  for free downloads? I know I am, so I created another text.dat string (reusing Unused300), for notification of free-download files where no credits are awarded to the uploader. Also updated the default colors of the existing DownloadUserMsg string to have a little more variety.

Not sure why ftpsrvr.c isn't using user_downloaded_file(). That copy/pasta should be eliminated later.