man: reorder/add sections to systemd.exec(5) (#7412)

man: reorder/add sections to systemd.exec(5) (#7412)

The long long list of settings is getting too confusing, let's add some
sections and reorder things in them.

This makes no changes regarding contents, it only reorders things,
sometimes reindents them, and adds sections that made sense to me to
some degree.

Within each sections the settings are ordered by relevance (at least
according to how relevant I personally find them), and not
alphabetically.