default_value defaults to nullptr.

Convenient method that behave differently than Read, and that will be used by
tag scanners.

This method will return in case of error, if the whole data is read or is EOF
is reached.

Fixes deadlock on small responses.

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

Fixes a problem with the "curl" input plugin: IsEOF() always returns
true because the "open" flag was cleared by
CurlInputStream::RequestDone() when end-of-stream was reached.  This
flag stays false even when seeking to another position has succeeded.

This patch resets the "open" flag to true after seeking successfully.

Fixes -Winconsistent-missing-override (clang 3.6).

Make sure that the log callback is installed.

Simplify the definition of many build options.

While seeking, metadata must not be updated.  ResponseBoundary() was
added in MPD 0.19.1, but I forgot to add the IsSeeking() check there.
This caused the "seekable" flag to reset.

Fixes crash in the "audiofile" decoder while logging the seek error.

Fixes playback of redirected streams.