INT 2F - AVATAR.SYS - SET DRIVER STATE
        AX = 1A21h (AL='!')
        DS:SI -> command string with one or more state characters
        CX = length of command string
Return: CF set on error (invalid subfunction)
        CF clear if successful
Note:   the characters in the state string are interpreted left to right, and
          need not be in any particular order

Values of state characters:
 'a'    activate driver
 'd'    disable driver
 'f'    use fast screen output
 'g'    always convert gray keys (+ and -) to function keys
 'G'    never convert gray keys
 'l'    convert gray keys only when ScrollLock active
 's'    use slow screen output
 't'    Tandy 1000 keyboard (not yet implemented)