Fixes aborted "load" commands due to SongLoader failure.

Obsolete class CloseSongEnumerator, which was a kludge.

This reverts commit d7d9dbd2 by
reimplementing it with the current MPD API.

3 years ago, I was wrong about the "embcue" plugin being able to
replace this one, because "embcue" reads a tag named "CUESHEET", while
this plugin reads the "CUESHEET" FLAC metablock.  There's an important
difference between those two!

Only ignore indexes after first nonzero, preserving pregaps but
appending them to the previous track instead of prepending to the
current. The first index of the first track is used for the start time
regardless of its number. Unneeded bool last_updated removed.

Use the first INDEX in each TRACK section, instead of the last, for the
start time. This preserves the original CD layout (including gaps
between tracks), and avoids skipping sections of songs in more exotic
cuesheets (eg musical suite tracks).

Fixes 0004355 and 0003359

Type-safe wrapper for uri_get_suffix().

The old struct config_param remains only for top-level string options.

The definition of the playlist_plugin struct member of the embcue
plugin was incorrect.