Adds a new branch to a transaction.

    Format

      SYS$START_BRANCH  [efn] ,[flags] ,iosb ,[astadr] ,[astprm]

                        ,tid ,tm_name ,bid [,[timout], [acmode],

                        [tx_class]]

    C Prototype

      int sys$start_branch  (unsigned int efn, unsigned int

                            flags, struct _iosb *iosb, void

                            (*astadr)(__unknown_params), int astprm,

                            unsigned int tid [4], void *tm_name,

                            unsigned int bid [4],...);