INT 2F U - NETWORK REDIRECTOR - OPEN EXISTING REMOTE FILE
        AX = 1116h
        ES:DI -> uninitialized SFT
        SS = DOS DS
        SDA first filename pointer -> fully-qualified name of file to open
        STACK: WORD file open mode (see INT 21/AH=3Dh)
Return: CF set on error
            AX = DOS error code (see INT 21/AH=59h)
        CF clear if successful
            SFT filled (except handle count, which DOS manages itself)
        STACK unchanged
Note:   called by DOS 3.1+ kernel
SeeAlso: AX=1117h,AX=1118h,AX=112Eh,INT 21/AH=3Dh,INT 21/AH=60h