Simplify and document parts of linux text system code (#9443)

I mainly focused on improving the `font_id` function, see the
description of e286483262e525c64c29d3747838ea9d2bac7bad for more
details. The rest are some drive-by changes I could not resist to.

When I am right about af4d6c43ce87ff5f16ad39c223e13326550010dd, someone
with a Mac could change it there as well.

This PR is probably best reviewed commit by commit :)

cc @gabydd @h3mosphere

Release Notes:

- N/A

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Signed-off-by: Niklas Wimmer <mail@nwimmer.me>