NAME
        AddPart -- Appends a file/dir to the end of a path (V36)

   SYNOPSIS
        success = AddPart( dirname, filename, size )
        D0                   D1        D2      D3

        bool addpart( strptr, strptr, ulong )

   FUNCTION
        This function adds a file, directory, or subpath name to a directory
        path name taking into account any required separator characters.  If
        filename is a fully-qualified path it will totally replace the current
        value of dirname.

   INPUTS
        dirname  - the path to add a file/directory name to.
        filename - the filename or directory name to add.  May be a relative
                   pathname from the current directory (example: foo/bar).
                   Can deal with leading '/'(s), indicating one directory up
                   per '/', or with a ':', indicating it's relative to the
                   root of the appropriate volume.
        size     - size in bytes of the space allocated for dirname.  Must
                   not be 0.

   RESULT
        success - non-zero for ok, FALSE if the buffer would have overflowed.
                  If an overflow would have occured, dirname will not be
                  changed.

   BUGS
        Doesn't check if a subpath is legal (i.e. doesn't check for ':'s) and
        doesn't handle leading '/'s in 2.0 through 2.02 (V36).  V37 fixes
        this, allowing filename to be any path, including absolute.

   SEE ALSO
        filepart(), pathpart()