Passing a void pointer is unsafe.

Move code from AlsaMixerPlugin.

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

This deadlock was a regression by commit 8e38b4f8.  Since we currently
can't resolve this, let's revert the commit, and add a GLib specific
workaround for the build failure.

This is safer, and works without epoll().  Fixes a build failure with
uClibc, which does not support epoll().

Replaces GLib's g_ascii_strcasecmp().

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Simplify the API, don't use GLib specific integer type.

Use libasound's polling functions, implement a bridge to GSource /
GPollFD and send idle events to clients when an external program
changes the ALSA mixer volume.

Better error handling.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

This allows the mixer object to access its associated audio output
object.