Legacy downloader requests have recieved a little TLC.

- Catch buffer overrun opportunities and fail early.
- Add #define MORELEGACYDOWNLOADER for the equivalent of MOREFILENEEDED, but disabled for now because honestly we really shouldn't be encouraging people to use this thing by making it support 255 WADs at once, but also because that'd be MISERABLE to test
- Add a menu report for when legacy downloader attempts fail