Check opus_decoder!=nullptr instead.

Commit d42c0f1d added an OS X-specific
method of calling mpd_main_after_fork(), which uses Grand Central
Dispatch. Since this uses a block literal, it breaks compilation on
compilers which don't support the block extension, e.g. non-Apple
compilers. This affects users on older OS X releases with GCD (which
depend on older Clang releases, or Apple GCCs, which don't support the
C++11 features MPD needs); or which don't support GCD at all (10.5 and
lower).

This patch changes the #ifdef so that the non-GCD code is used
as it was on OS X before this patch if blocks aren't available, via
checking __BLOCKS__ macro.

The old formula calculates the output buffer size with "regular"
rounding (to the nearest integer), however sometimes, that is
insufficient and the last sample cannot be resampled.  This causes
audible distortions.  By changing the formula to consider the worst
case (always round up), this problem is eliminated.

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.

Eliminate one implicit std::string copy.

On "list albumartist", songs that have no AlbumArtist tag will use the
Artist tag.  However, if AlbumArtist is disabled via
"metadata_to_use", the TagBuilder::AddItem() call is ignored, and
PrintUniqueTag() attempts to print a nullptr string.

This commit fixes the problem by attempting the fallback only if
AlbumArtist is not disabled.

When uri_apply_base() was moved from db/upnp/Util.cpp to
util/UriUtil.cpp, the parameter order was changed, however without
swapping the parameters in the ContentDirectoryService constructor.

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

Fixes playback of redirected streams.

Use the new uri_get_suffix() overload that removes the query string.