gdbint.texinfo fixed, Index added

[Eli Zaretskii]

FYI: I've just committed a large chunk of mundane changes to 
gdbint.texinfo.  I don't want to bore you-all to death by posting
a 145KB patch here, but the upshot is that now gdbint.texinfo has
a decent Index, and the markup is, by and large, fixed.  (This
includes the @itemize lists where the @item lines were not alone
on their lines, which I promised to fix some time ago.)

I tested this with Texinfo 4.0, so if someone could try v3.12,
please do.

Enjoy the lifted-face manual ;-).