Function copy_file

Copy contents of file @source to file @dest with mode @mode. * * If @tmpfilefd is < 0, copy_file unlinks @dest and then opens a new * file with name @dest. * * Otherwise, copy_file writes to and closes the provided file * descriptor. * * In either case, if --xattrs are being preserved, the dest file will * have its xattrs set from the source file. * * This is used in conjunction with th