configure.ac: prepare for 0.19.3
input/Open: expose input_domain
input/AsyncInputStream: set Error when seeking unseekable

Fixes crash in the "audiofile" decoder while logging the seek error.
lib/upnp/ContentDirectoryService: swap uri_apply_base() parameters

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.
db/upnp: fix valgrind warning
decoder/ffmpeg: support opus
Merge branch 'v0.19.x'
db/upnp/Object: apply coding style
db/upnp/Directory: move checkobjok() to class UPnPDirObject
db/Count: include cleanup
tag/Set: do AlbumArtist/Artist fallback only if AlbumArtist is not disabled

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.
db/upnp/Directory: apply coding style
db/upnp/Directory: TitleToPathSegment() returns rvalue reference

Eliminate one implicit std::string copy.
input/curl: ignore ResponseBoundary() while seeking

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.
test/FakeDecoderAPI: dump bit rate
decoder/audiofile: fix bit rate calculation
pcm/SoxrResampler: round output buffer size up

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.
Main: fix compilation on OS X using non-Apple compilers

Commit d42c0f1dc5063d50a62817b63a1c2a4507c46071 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.
decoder/opus: add constexpr output_buffer_frames
decoder/opus: eliminate flag "found_opus"

Check opus_decoder!=nullptr instead.