This tiny amount of overhead allows omitting the MusicBuffer in
Clear().

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.

If an early exception gets caught (e.g. from
AllocatedPath::FromUTF8Throw()) before
DecoderControl::CommandFinishedLocked() is called, the decoder thread
would go in an endless loop, because DecoderCommand::START is still
set.

Closes #118

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

This repairs the damage to commit 74dbaade done by commit
b3f5b493

 https://bugs.musicpd.org/view.php?id=4595

Catch ENOTDIR exceptions and in that case, try all matching decoder
plugins which implement the "container_scan" method.

 https://bugs.musicpd.org/view.php?id=4561

Replaces decoder_data() and others.

Code simplification.

Be exception-safe.

Prepare full C++ exception support in the decoder thread.

Fixes memory leak after stream failure.  See
https://bugs.musicpd.org/view.php?id=4562

This reverts commit 995cd954.  It was
a bad idea because it caused assertion failures when a decoder plugin
aborted with an exception.