INT 21 - Attachmate Extra - SEND KEYSTROKES TO HOST WINDOW
        AH = A3h
        AL = window number (1-4=host B-E)
        CX = 0001h
        DS:BX -> keystroke buffer
        DL = zero if keystroke buffer contains host function code,
             non-zero if keystroke buffer contains ASCII character
Return: CX = zero if character sent, non-zero if not
        BX incremented if CX=0

Values for host function code:
        00h=reserved    10h=PF16        20h=Clear       30h=SysRq
        01h=PF1         11h=PF17        21h=Print       31h=ErInp
        02h=PF2         12h=PF18        22h=Left        32h=ErEof
        03h=PF3         13h=PF19        23h=Right       33h=Ident
        04h=PF4         14h=PF20        24h=Up          34h=Test
        05h=PF5         15h=PF21        25h=Down        35h=Reset
        06h=PF6         16h=PF22        26h=Home        36h=DevCncl
        07h=PF7         17h=PF23        27h=Fast Left   37h=Dup
        08h=PF8         18h=PF24        28h=Fast Right  38h=FldMark
        09h=PF9         19h=Alt on      29h=Bksp        39h=Enter
        0Ah=PF10        1Ah=Alt off     2Ah=Insert      3Ah=CrSel
        0Bh=PF11        1Bh=Shift on    2Bh=Delete
        0Ch=PF12        1Ch=Shift off   2Ch=Backtab
        0Dh=PF13        1Dh=PA1         2Dh=Tab
        0Eh=PF14        1Eh=PA2         2Eh=Newline
        0Fh=PF15        1Fh=PA3         2Fh=Attn