INT 21 - LANtastic - SET DOS SERVICE VECTOR
AX = 5FE1h
ES:BX -> FAR routine to call when DOS services are available
Return: CF clear if successful
CF set on error
AX = error code
Note:   new handler must chain to previous handler as its first action

See also: 215FE0

215FE1 - LANtastic - SET DOS SERVICE VECTOR