Separate the indices in gdbint.texinfo into 2 nodes, to avoid file-name clashes.

Separate the indices in gdbint.texinfo into 2 nodes, to avoid file-name clashes.

    * gdbint.texinfo: Remove @syncodeindex directives that put all the
    indices into a single node "Index".  Instead, join function and
    variable indices into a single index, separate from the concept
    index.  Requested in
    http://sourceware.org/ml/gdb-patches/2012-07/msg00146.html.
    (Top): Update the top-level menu: instead of one "Index" there are
    now separate entries "Concept Index" and "Function and Variable
    Index".
    (Concept Index, Function and Variable Index): New nodes.
    (Index): Node deleted.