Get Selector Increment Value

The DPMI functions Allocate LDT Descriptors (Int 31H
Function 0000H) and Allocate DOS Memory Block (Int 31H
Function 0100H) can allocate
an array of contiguous descriptors, but only return a
selector for the first descriptor.  The value returned by
this function can be used to calculate the selectors for
subsequent descriptors in the array.

Call With:

   AX     = 0003H

Returns:

   Carry flag= clear (this function always succeeds)
   AX     = selector increment value

Notes:

o  The increment value is always a power of two.