#22 dos_open() should detect and reject headerless files

[Brian Ruthven]

During creation of the readaline() interface, some strange results were obtained. This involved rounding up the length of the file being read to the next 128-byte boundary. It transpired that the file being read had no header (a mistake when it was copied to the disk image), but dos_open() should probably detect and warn about this, possibly blocking the open() with an ENOHDR or something like that.