The wrong variable was used.  This has been broken by commit
4eeea640 seven years ago - and nobody as noticed, d'oh!

libavahi-client uses this one to schedule events immediately.
However, CoarseTimerEvent may be called too late, and cause timeouts.

Deprecated in C++17.  Since C++17, one is supposed to declare those 5
types manually.

This was a no-op previously, but this operation makes no sense.

The existing implementation has been utterly broken forever; I cannot
explain what it actually does, but it doesn't do what the
documentation says.

Use MoveRange() instead of MoveId().

Now, "+0" means "right after the current song" and "-0" means "right
before the current song".  Mnemonic: there are zero songs between the
current song and the newly added song.

A similar feature was present long ago in MPD, but was deprecated in
version 0.16 because the implementation was broken.  This commit
re-adds the feature in a way that's well-defined and not broken.

Close https://github.com/MusicPlayerDaemon/MPD/issues/1221

Validate early, so we avoid the rollback if an error occurs.

Skip the LookupRemoteTag() call if the MoveId() call fails.

Eliminates lots of implicit std::string temporaries.