DOSAMP: Now with runtime file selection

• Windows builds (but not Windows NT console builds) can now accept new files to play via drag & drop from your Windows shell.
• Running DOSAMP without a file to play now prompts for the file. If you do not select a file, then the player just does nothing, but you can always select another file.
• A terminal-based file selection UI is now available. You can type in the path of the WAV file to play with backspace, CTRL+W word erasure, and Bash-inspired tab completion on Linux, MS-DOS, and Windows systems where the "open file dialog" common control is not available.