Int 21 Fn 61  U - OS/2 V1.x Fapi - Os/2 File System Join/subst             [O]

   AH = 61h
   BP = 6467h ("dg")
   AL = function
       00h list
       01h add
       02h delete
   BX = drive number
   CX = size of buffer
   SI = type (0002h JOIN, 0003h SUBST)
   ES:DI -> buffer

Return: ???

Notes: used by JOIN and SUBST to communicate with the OS/2 file system
   also supported by OS/2 v2.0+ Virtual DOS Machines