This fixes compatibility with Plex DLNA.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/851

Closes https://github.com/MusicPlayerDaemon/MPD/issues/841

Fixes the "Unrecognized URI" error with the udisks storage plugin,
which is caused by the kludge in UdisksStorage::MapUTF8().

The `db->close()` call was a `nullptr` dereference because the `db`
variable had already been moved.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/839

This method gets called a lot during MPD startup, via FindChild() and
directory_load_subdir(), so this is worth optimizing at the expense of
code readability.

This speeds up MPD startup by 10%.

Found with readability-uppercase-literal-suffix
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Merge some duplicate code in a central library.

Add a runtime sanity check to avoid the assertion failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

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

Expect OnSocketReady() to cancel events.  If it returns false, the
SocketMonitor may be destructed already.  This fixes a use-after-free
bug in the "httpd" output plugin.

Fixes a memory leak, or an assertion failure in the debug build.

Fixes assertion failure.

`light_song.Get()` could cause an assertion failure because the
`Manual<>` object must not be used if uninitialized.

Regression by commit ebc006ab

GCC 9's libstdc++ is unable to use forward-declared types as
std::vector item because the compiler wants to resolve `noexcept()` on
the item destructor.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/456

Since commit cc64c715, UpdateService::Enqueue() throws
ProtocolError(ACK_ERROR_UPDATE_ALREADY) instead of returning 0.

This function implements the "group" feature on top of the
Database::Visit() method.