More details from text.dat strings when removing files or credits

Don't display "FREE" in file listings for 0 byte files.

When removefcdt() is called for an unknown file uploader, use the new
UnknownUploader text.dat string to display the username and filename.

When prompting to remove credits, include the username from which to remove
the credits.

These usability improvements came from watching Deuce struggle to remove
malicious uploaded-files and credits from (presumably Guest) user. I should
watch sysops manage their BBSes more often.