Don't iterate the std::list each time.

Commit 1f119597 allowed modifying the "quit" attribute from any thread,
and thus the assertion may fail spuriously.  This assertion is too
strict for the relaxed use of "quit".  Let's remove it and move the
"quit" check to before the SockedMonitor::Dispatch() call.

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.