This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Fixes a deadlock caused by commit
31ab78ae.  That commit was not
actually bad - just these two calls have always been bad, which went
unnoticed for a long time.

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!

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

Don't use GLib.

Simplify the definition of many build options.

Use the new uri_get_suffix() overload that removes the query string.

Simplifies a lot of code, because we don't need to return both the
SongEnumerator and the InputStream.

Merge duplicate code.

Merge some duplicate code.

Replaces g_uri_parse_scheme().