More robust crux-copy-file-preserve-attributes - #100

crux-copy-file-preserve-attributes suffers from several problems:

• A new buffer is always opened without correctly taking into account the value of universal argument
• If filename is different, the new opened buffer contains the source file (destination file in same directory) or an empty buffer (destination file in other directory)
• If the destination chosen by user is a complete path (with filename) in a non-existent directory, the function fails with error "Opening output file: No such file or directory"

So I rewrote the function in a more robust way to to limit mishandled cases.