Compatibility: 3.0+
Expects: AX    5c01H
CX:DX file offset from start of file    (CX * 65536)+DX
SI:DI length in bytes of region to lock (SI * 65536)+DI
Returns: AX    error code if CF is set to CY
Info: This function unlocks access to a region of the file which was
previously locked.

See fn 5c00H for more details on file locking.

Notes:   The offset and length must match exactly with the parameters
used when the lock was applied.

  It is important that all locks be removed from a file before a
program is terminated.

  It is recommended that you unlock as soon as possible.  Always
try to lock, access the file, and unlock in one operation.

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DOS Fn 5c01H: Unlock File Access