config: allow spreading config over multiple files
This is the Go-equivalent to
https://lists.sr.ht/~sircmpwn/sr.ht-dev/patches/47657
This commit enables spreading the config in /etc/sr.ht - and, crucially,
_only_ in /etc/sr.ht - over multiple .ini files.
If a file config.ini is found (either in current or parent directory, or
/etc/sr.ht) it (and only it) is loaded and any other ini files are
ignored. To utilize multiple configs, they must be in /etc/sr.ht, and
none of them must be called config.ini.
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.