Improved support for added events.

This call is not thread-safe.

Client::Close() installs a TimeoutMonitor, which is not something we
should do during shutdown.

For std::find_if().

Move code from AlsaMixerPlugin.

Now that the remaining known bugs in poll() implementation are fixed,
we can go on without the GLib implementation.

Fixes more thread-safety bugs.

Add a Page queue to class HttpdOutput, and use DeferredMonitor to
flush this queue inside the IOThread.  This fixes a thread-safety
issue: much of EventLoop is not thread-safe, and the httpd plugin
ignored that problem.

Do all I/O in the IOThread and not in the main thread.  This solves an
upcoming deadlock problem.

Don't let the server care for client problems.

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.