INT 31 P - DPMI 0.9+ - FREE REAL MODE CALLBACK ADDRESS
        AX = 0304h
        CX:DX = real mode callback address
Return: CF clear if successful
        CF set on error
            AX = error code (DPMI 1.0+) (8024h) (see AX=0000h)
Note:   the limited DPMI host built into Phar Lap's 286|DOS-Extender v2.5 does
          not support this function
SeeAlso: AX=0303h

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