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.

Enables using range-based "for".

The string "base" is understood by locate_parse_type(), but not by
listAllUniqueTags().  The special tag type LOCATE_TAG_BASE_TYPE causes
a crash in PrintUniqueTag().

Lists files and directories.  Supports storage plugins.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Zero means "unknown".

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.