Allow configurable file size limits for file transfer protocols

Some (traditional/modem) file transfer protocols and/or drivers don't support transferring files larger than certain sizes (e.g. 2GB or 4GB due to 32-bit limitations). Allow each configured protocol in SCFG->File Options to (optionally) have a specified max file size so we can fail these file transfer (download) attempts gracefully with a nice user-friendly error message.