godoc: improve the source layout to enable copying code...

godoc: improve the source layout to enable copying code without line numbers.

The following changes were required to achieve this:

* Modified the css used by the godoc tool to reassign the generally used styles for the pre tag to a class.
* Adjusted the routine which builds source pages so it uses a slightly differnt style using a table to seperate the line numbers from sources.
* Updated other places which used the pre tag to also include the new class.

The aim of the style changes was to leave the font styling on the pre, but move the specific framing to a class enabling variations in the future.