Improve network drawings

[obonaventure]

The ebook should mainly use tikz for drawings because they are easier to maintain than the current PNG files extracted from old slides.

However, using circles for hosts and rectangles for routers is not visually friendly. We should define a library of icons that can be reused throughout the ebook.

Here are two useful starting points:

https://github.com/moepinet/moeptikz (library of networking icons)
https://www.ctan.org/tex-archive/graphics/pgf/contrib/tikzpeople (tikz people library, mainly useful for the security section)

You can suggest other tikz solutions

[obonaventure]

There is a first step in the tikz.preamble in the 4ed branch, but it needs to be improved ad the host shape should be built from a pgfshape and not using picture