This commit enables spreading the config in /etc/sr.ht - and, crucially,
_only_ in /etc/sr.ht - over multiple .ini files.
As before, if a file ./config.ini is found, it (and only it) is loaded
and any config in /etc is ignored.
Spreading the config over multiple files will make it much easier to
create containerized versions, where e.g. different secrets can be made
available in different files, but rendering it all into one big file
would require some preprocessing.
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The behavior of not loading ./*.ini is my suggestion. In my opinion
loading ./config.ini is more of a dev environment thing, and I have
various .ini files in the base folder for different scenarios, which
would clash. But I am open to suggestions.