This mostly affects the Pulse output plugin which needs to "cork" the
stream (closes #278).

Closes #277

These are used by DSD-streaming servers.  For example, MiniDLNA uses
"audio/x-dsd".

Error message sent to client was "basic_string::_M_construct null not
valid" due to passing nullptr to the std::string constructor.

Regression caused by commit 386688b8

Closes #258

Fixes "search already in progress" errors.

av_register_all() was deprecated in
FFmpeg/FFmpeg@0694d8702421e7aff1340038559c438b61bb30dd

When switching to another song manually, the player checks if the
decoder is already decoding that song; if so, it will attempt to reuse
it by seeking it to the new position.  That however fails if the
decoder is not seekable (e.g. a radio stream) which leaves the user
unable to switch to that song with the bogus error message "Not
seekable".

There was a discrepancy between what was written to the buffer and the
size returned by pcm_dsd_to_dop(): the "for" loop uses num_frames/2,
rounding down, while the return value is num_samples which is
num_frames*channels, without rounding.  This could cause undefined
data at the end of the destination buffer if the source buffer size
was not aligned to multiples of 8 bytes (4 DSD bytes per channel).

The latter however can occur in the 0.21 branch after commit
a06bf388

Closes #233

Fixes crash due to "pure virtual method called" in the "mms" input
plugin.  Closes #253

Closes #247

Without setting the "ready" flag, the caller will wait in WaitReady()
forever, locking up MPD.  Closes #252

lockfree library used by ALSA output plugin is part of Boost from version 1.53,
so this can be theoretically the lowest required version, however
there are issues which are resolved from 1.54 onwards.

First commit for issue #69

Instead of stopping playback (due to seek time overflow), reject the
seek command.  Closes #240

Relative negative values (with "seekcur") are still allowed, and MPD
will fix the resulting position if it turns out to be negative.  But
the "seek" and "seekid" commands use an unsigned time stamp which must
not be negative.

With Grand Central Dispatch used in Main.cxx, debug builds on macOS
crash as the IsInside() assertion gets triggered in the event loop. As
a simple fix, usage of GCD is removed. Plugging and unplugging
headphones or changes of the default output device was tested without
issues. Whatever the original commit tried to fix by GCD probably does
not need fixing anymore.

Now that all build failures have been fixed, we can enable the
feature.

Fixes two build failures with libnfs on Windows.