Int 2F Fn 120C  U - DOS 3+ Internal - Open Device And Set Sft Owner/mode   [D]

   AX = 120Ch
   SDA current SFT pointer -> SFT for file
   DS = DOS DS
   SS = DOS DS (must be using a DOS internal stack)

Return: ES, DI, AX destroyed

Notes: invokes "device open" call on device driver for SFT
   changes owner of last-accessed SFT to calling process if it was opened
     via FCB
   called by network redirectors such as MSCDEX