Define the replacements for generic families on Linux

FontConfig does not know much about our fonts, and falls back to the font version, when it cannot find any other criterion to decide which font to use (see tor-browser#41043 (closed)).

So, we want to make sure that at least sans-serif, serif and monospace are the fonts we want (currently, Arimo, Tinos and Cousine).

This is needed for example for tor-browser#41116 (closed): we have patched system fonts, but Firefox does not double check their values with the font replacement system, and queries FontConfig for the font to use directly.

I think that patching the fonts.conf is the correct solution for this use-case.