Fixes shutdown memory leak.

This is implicitly done by the destructor.

Fixes shutdown memory leak.

The output plugin shall decide whether to insert silence or do nothing
at all.  The ALSA output plugin has already implemented this.
Inserting silence is not necessary or helpful for some plugins, and
may even hurt them (e.g. "recorder").

Eliminates unnecessary mutex locks.

Closes #181

Wired to Filter::Flush().

Closes #153

This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

By API contract, this parameter must not be zero.

Adds missing checks to several plugins.

Fixes another crash bug caused by commit
64d141f7

Fixes crash bug caused by commit
64d141f7

Caused by commit 64d141f7

This wasn't a serious memory leak, because after a mount failure, MPD
would abort anyway, which is subject to the next commit.