"DEFAULT" is a bad name - all it says is that it's the default value,
but it doesn't say what it means.  The name NOTICE mimics the syslog
level.

The ones in std have overloads for const char/char.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-use-override
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Regression by commit bbdf2dcf

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

This is the final piece for full ID3/APE support in archives.

Don't open the ZIP file again and again for each song file.

This commit finally allows the database update to scan remote files,
e.g. using the smbclient storage plugin.  However, it is not yet
possible to configure that, therefore the feature is not accessible
yet.

Keep only a reference.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.