INT 21 - DoubleDOS - SET TIMESHARING PRIORITY
        AH = E9h
        AL = 00h visible program gets 70%, invisible gets 30% (default)
           = 01h visible program gets 50%, invisible gets 50%
           = 02h visible program gets 30%, invisible gets 70%
           = 03h Top program gets 70%, bottom program gets 30%
           = 04h Top program gets 30%, bottom program gets 70%
           = 05h get current priority
                Return: AL = priority setting
Note:   identical to AH=F9h
SeeAlso: AH=EBh"DoubleDOS",AH=F9h"DoubleDOS"