Use the snd_pcm_t only in the IOThread, and reuse code that is
well-known to work.

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Same fix as in commit 1c904000

default_value defaults to nullptr.

Fixes deadlock on small responses.

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

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

Make sure that the log callback is installed.

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 playback of redirected streams.

Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

This special case was useless.  Fixes the dreaded "Unrecognized URI"
error message when a file does not exist.

The second time init was called, http_200_aliases pointed to a freed pointer
and leaded to a SEGFAULT.