Int 2F Fn 1A21  - Avatar.sys - Set Driver State                            [V]

   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

See Also: AX=1A3Fh

Values for 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)