INT 21 - LANtastic v3+ - GET USERNAME FROM ACCOUNT FILE
        AX = 5FB2h
        BX = username entry index (0 for first)
        DS:SI -> 16-byte buffer for username
        ES:DI -> ASCIZ server name in form "\\machine"
Return: CF clear if successful
        CF set on error
            AX = error code
        BX = next queue entry index