INT 31 P - DPMI 0.9+ - FREE DOS MEMORY BLOCK
        AX = 0101h
        DX = selector of block
Return: CF set if successful
        CF set on error
            AX = DOS error code (07h,09h) (see INT 21/AH=59h)
Notes:  all descriptors allocated for the block are automatically freed
        DPMI 1.0+ automatically zeros any segment registers containing a
          selector freed by this function
        not supported by MS Windows 3.0 in Standard mode
SeeAlso: AX=0102h,AX=0502h