Instead of passing tag and group, pass an array of tags.  To support a
nested return value, return a nested std::map of std::maps.  Each key
specifies the tag value, and each value may be another nesting level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/408

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.

Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

That way, each plugin can decide to implement it better.

Remove them as parameters from `db_selection_print()`.

This enforces the "not nullptr" rule.

Closes #172

This code (added 7 years ago with commit b233c145) has been obsoleted
by the SongLoader class (added 3 years ago).

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

The "tagtypes" command now has several sub commands which can be used
to edit that mask.

Implementations shall use exceptions instead.

Fixes compiler warning.