Wrong anchor adjustment in tableheader.js

If a page contains an internal anchor link, i.e. <a href="#foo">, and the user clicks on it, tableheader.js scrolls a bit backwards in order to make room for the header above the link target.

The code assumes that the anchor is a &;td> - I don't know why that is. If the anchor isn't a <td>, scrollLocation becomes negative, and the page is scrolled to the top, right after in has scrolled to the anchor further down the page. In other words, using anchors on a page using tableheader.js does not work.

Also, the adjustment code is inside the tracker() function that is invoked once per table on the page, but the adjustment is only done once using the height of the header of the first table on the page (after that prevAnchor == location.hash), even though the anchor is inside another table or not in a table at all.

This patch makes tracker() only adjust the scroll if the anchor is inside the current table, and when the header is displayed (it's not when the anchor is e.g. in the very last row).