INT 15 - DESQview v2.26+ - "ASSERTMAP" - GET/SET MAPPING CONTEXT
        AX = DE17h
        BX = 0000h      get current mapping context without setting
             nonzero    set new mapping context
Return: BX = mapping context in effect before call
Notes:  mapping contexts determine conventional-memory addressability; setting
          a mapping context ensures that the associated program and data areas
          are in memory for access.  Usable by drivers, TSRs and shared
          programs.
        caller need not be running under DESQview
        this API call may be made from a hardware interrupt handler
SeeAlso: AX=DE21h,INT 2F/AX=1685h