See code comment.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/629

This optimization is useless because sane pthread_cond_signal()
implementations check the number of waiters and do not invoke a system
call if there are none.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

This requires re-adding the reference to struct DecoderControl, which
was removed recently by commit
9f14e7a9

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

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

Previously, both methods accessed a lot of attributes which require
mutex protection.

Obsoletes the same variable from AudioConfig.cxx.

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

The state ERROR can only be entered during decoder initialization, and
Seek() may only be called after initialization has been finished.