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

Preparing to move logger calls out of lower-level libaries, and
propagating error details to the caller instead.

Regression by commit bbdf2dcf

Allow forward-declaring it.

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.