Switch graphviz files to use HTML-style labels

With recent versions of graphviz, generation of the diagrams in the documentation
fails with:

 /usr/bin/dot -Tpng -oxml/x-architecture.png dot/x-architecture.gv
 Warning: flat edge between adjacent nodes one of which has a record shape - replace records with HTML-like labels
   Edge xserver -> comp
 Error: getsplinepoints: no spline points available for edge (xserver,comp)
 Error: lost xserver comp edge
 Error: lost xserver comp edge
 Error: lost comp xserver edge
 Error: lost comp xserver edge

http://www.graphviz.org/content/i-havent-been-able-render-these-files-graphviz-226

 indicates
that the error message basically means that the authors of graphviz consider record-style
labels to be deprecated and are no longer fixing errors with them. This patch changes
the labels to be in the HTML style, which seems to require duplicating style between all
the nodes, but it's not like these files are often edited.

The result is not exactly the same but is quite similar.

Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
Tested-by: Armin Krezović <krezovic.armin@gmail.com>