various: use new config loader instead of config_parse_config_file()

various: use new config loader instead of config_parse_config_file()

This means the main config file is loaded also from /run and /usr.

We should load the main config file from all the places where we load drop-ins.

I realize I had a giant blind spot: I always assumed that we load config files
from /etc, /run, /usr/local/lib, /usr/lib. But it turns out that we only used
those paths for drop-ins. For the main config file, we only looked in /etc. The
docs actually partially described this behaviour, i.e. most SYNOPSIS sections
and some parts of the text, but not others.

This is strange, because 6495361c7d5e8bf640841d1292ef6cfe1ea244cf was completely
bogus with the behaviour before this patch. We had a huge discussion before it
was merged, and clearly nobody noticed this. Similarly, in the previous version
of the current pull request, we had a long discussion about the appropriate
order of directories, and apparently nobody noticed that there was no order,
because only looked in one directory. So the blind spot seems to have been
shared.

Also, systemd-analyze cat-config behaved incorrectly, i.e. its behaviour matches
the new behaviour.

Possibly, in the future it'll make it easier to add support for --root.