Restores the features from the previous draft commands "findin" /
"searchin".

This reverts commit a577944a.

Will be replaced by new options for the old commands "search" and
"find".

This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients.  On request, I
add it to the protocol again.

Requires UNIX domain socket connection.

(Maybe it should be "A new message is indicated...", I don't know.)
(                                ^^                               )

For simpler seeking within current song.

Sorts remaining songs by priority.  This can be used for the
much-demanded "queue feature".

Adds mixrampdb and mixrampdelay commands.  Reads MIXRAP_START and
MIXRAMP_END tags from FLAC files and overlaps instead of crossfading.

Same for "listplaylistinfo".

This command prints a list of decoder plugins and their suffixes /
MIME types.

When the range end is missing, then the maximum possible value is
assumed.

The new command "replay_gain_mode" allows the user to switch the
replay gain mode on-the-fly.  No more mpd.conf editing.

"rescan" is the same as "update", but it discards existing songs in
the database.

The update job id is positive.  When used in a command list, MPD
prints one job id for each "update" command.

This command has been deprecated more than 5 years ago (0.10.0).  Its
implementation is a kludge, let's remove it now.

Some clients have visual feedback for "database update is running".
Using the "database" idle event is unreliable, because it is only
emitted when the database was actually modified.  This patch adds the
"update" event, which is emitted when the update is started, and again
when the update is finished, disregarding whether it has been
modified.