#66 ENOSYS overlaps with +3DOS error numbers

[Brian Ruthven]

DOS_ERR is currently set to 64. Since ENOSYS was added, although unlikely it will be returned, it's value coincides with the +3DOS error no 25 "End of file" (64+25=89). There are a number of things we can do here:

1. Do nothing. Accept that ENOSYS is rare, and assume that 89 means "End of file".
2. Set DOS_ERR to 128, lifting the DOS error numbers clear of ENOSYS. A variant would be to add 100, making the error numbers somewhat easier to decode by eye. The springboard code does an arithmetic add, not a bitwise or, so this is perfectly possible.
3. Move ENOSYS to a different number.

My personal preference (at the moment!) is to increase DOS_ERR.