Kludge for libavahi-client quirk.

Add the debug-only flag "virgin" which gets checked by assert()
calls.  Fixes assertion failures when using zeroconf/avahi.

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.

Debug-mode workaround for bogus assertion failure.

Add a flag that indicates whether the EventLoop is currently "busy".
As long as that flag is set, it does not need to be woken up - we can
simply add the DeferredMonitor to the list, and it will be caught by
EventLoop very soon.  This eliminates nearly all of the
DeferredMonitor overhead when compared to IdleMonitor, rendering
IdleMonitor mostly obsolete.

Reduce DeferredMonitor overhead.

Improved support for added events.

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.

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.

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().

The DynamicFifoBuffer methods never return nullptr when the buffer is
empty or full; instead, they return an empty buffer.  This bug caused
an endless loop.

Switch to the C++ version.

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

No GLib memory allocation.