Creates a virtual mailbox device named MBAn and assigns an I/O
    channel to it. The system provides the unit number n when it
    creates the mailbox. If a logical name is specified and a mailbox
    with the specified name already exists, the $CREMBX service
    assigns a channel to the existing mailbox.

    Format

      SYS$CREMBX  [prmflg] ,chan ,[maxmsg] ,[bufquo] ,[promsk]

                  ,[acmode] ,[lognam] ,[flags] ,[nullarg]

    C Prototype

      int sys$crembx  (char prmflg, unsigned short int *chan,

                      unsigned int maxmsg, unsigned int bufquo,

                      unsigned int promsk, unsigned int acmode, void

                      *lognam,...);