Get/set mapping context

        Entry:
          AX = 1117h
          BX = 0000h    get current mapping context without setting
               Nonzero  set new mapping context

        Return:
          BX = mapping context in effect before call

        Notes:  This function differs from AX = DE17h 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