Adds a specified number of new virtual pages to a process's
    program region or control region for the execution of the current
    image. Expansion occurs at the current end of that region's
    virtual address space.

    Format

      SYS$EXPREG  pagcnt ,[retadr] ,[acmode] ,[region]

    C Prototype

      int sys$expreg  (unsigned int pagcnt, struct _va_range *retadr,

                      unsigned int acmode, char region);