Routine name: kjt_file_sector_list

Action: Retrieves the location of the (next) disk sector that a file occupies.

Input registers:	A  = Sector offset (within current cluster)
			DE = Cluster number

Output registers:       A  = Next sector offset
                        DE = Updated cluster
                        HL = Memory address of LSB of 4 byte sector location

How to use:

To obtain the list of sectors that a file uses, first call "kjt_find_file" with
the filename in HL as normal. On return, DE will be equal the first cluster that
the file occupies. Clear A (as we're starting at the first sector of this cluster),
and call "kjt_file_sector_list". On return A and DE are updated to the values required
for the next time the routine is called (so store them) and HL points to the lowest
significant byte of the sector address (Sector_LBA0). Copy the 4 bytes from HL to HL+3
to your sector list buffer and loop around, calling "kjt_file_sector_list" (supplying
it with the updated values for A and DE) for as many sectors as are used by the file
(simply subtract 512 from a variable holding the file size every call until variable is = < 0)

Notes: Obtaining the sector list is fast (it doesn't need to load the actual file data)
and using an obtained sector list to load the file in question is much faster than
the normal filesystem load routine.