#61 Expand dos state struct and array

[Brian Ruthven]

The dos_filenum struct is just an array of thread IDs, indexed by file number. It might be better for tracking and efficiency if this were an array of structs (including the thread id) but also could include the filename (as we had to dup it for the original open() call), and other information.

The address of this statically-declared struct could instead be passed back as the state information, rather than having to re-index into the array each time it is needed.