db: fix broken command "list ... group"

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 ae178c77bd, 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.

src/db/DatabasePrint.hxx

@@ -20,7 +20,7 @@
 #ifndef MPD_DB_PRINT_H
 #define MPD_DB_PRINT_H
 
-#include "tag/Mask.hxx"
+#include "tag/TagType.h"
 
 class SongFilter;
 struct DatabaseSelection;
@@ -44,7 +44,7 @@ db_selection_print(Response &r, Partition &partition,
 
 void
 PrintUniqueTags(Response &r, Partition &partition,
-		unsigned type, tag_mask_t group_mask,
+		unsigned type, TagType group,
 		const SongFilter *filter);
 
 #endif