Simply set the "quit" flag and wake up the thread.  This works even if
we're inside this thread.  Setting "quit" to a new value without mutex
protection is usually not safe, but good enough here.

EventLoop::AddCall() and EventLoop::AddIdle() are unsafe, because we
can't cancel those calls.

There is no advantage of using EventLoop::AddCall(), now that
DeferredMonitor is thread-safe.

Instead of creating a new eventfd for each DeferredMonitor instance,
reuse EventLoop's eventfd, and add a std::list to EventLoop that
manages the list of pending DeferredMonitors.  This std::list is
protected by the same mutex as the "calls" list.

The bottom line is: reduced overhead because the per-instance eventfd
was eliminated, slightly added overhead due to Mutex usage (but
negligible), and we're thread-safe now.

This subsystem is now good enough to replace EventLoop::AddCall().

Add the TagStream.cxx library, similar to TagFile.cxx, and use it to
load tags from song files inside archives.

Merge some duplicate code.

Got lost in commit c97685fe

Replaces decoder_plugin_from_suffix().

.. instead of decoder_plugin_from_suffix().  This reduces overhead by
walking the array only once.

Instead of using the first DecoderPlugin that supports the suffix, use
the first one that actually implements the "container_scan" method.

.. instead of decoder_plugin_from_suffix().  This reduces overhead by
walking the array only once.

If we have libyajl 2.0.1 (without a pkg-config file), our configure.ac
would assume this is the libyajl 1.0 API, because the function
yajl_alloc() exists in both.  This commit changes the library check to
the function yajl_parse_complete() which was removed in the 2.0 API.
This fixes build failure with libyajl 2.0.1.

We can assume that initgroups() would be a no-op in that case, however
initgroups() is not allowed for unprivileged users anyway.

Use the existing PcmDither library.

Instead of just adding a rectangular random value before shifting back
to the normal scale, use the existing PcmDither library.