Interactive links in the TOC go to the page start, not the section start

[Bruno Haible]

Hi,

PDF readers, like HTML document viewers, have the ability to position
the canvas in such a way that any specified line is at the top.
(DVI viewers like 'xdvi' and PostScript viewers like 'ghostscript'
don't have this ability.)

Texinfo-generated PDF files make use of this feature for hyperlinks
inside the document, generated from @xref or @ref. However, hyperlinks
inside the table-of-contents don't: they point to the start of the page.
This leads to reduced usability: After clicking on a link in the
table-of-contents, the user sees a page and then has to search through
the page: "where is the section that I wanted to read?"

How to reproduce:
1. Unpack texinfo-7.1.90.tar.xz
2. cd texinfo-7.1.90
3. ./configure
4. make
5. cd doc
6. make pdf
7. okular texinfo.pdf
   or: firefox ./texinfo.pdf
8. Go to page 19 (32 of 341) and click on "Section 2.5.3 [@settitle], page 14."
   The window is repositioned at section 2.5.3. Good.
9. Go to page ii (4 of 341) and click on "2.8.1 @titlepage".
   The window is repositioned at the beginning of page 18, before section 2.7.2.
   Not what I wanted. I wanted to go to section 2.8.1.

Bruno