The old struct config_param remains only for top-level string options.

Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

Replaces GLib's g_ascii_strncasecmp().

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

Replaces GLib's GError.

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.

Add a "mode" argument to open_cloexec() instead.

Added the "fd_util" library, which attempts to use the new thread-safe
Linux system calls pipe2(), accept4() and the options O_CLOEXEC,
SOCK_CLOEXEC.  Without these, it falls back to FD_CLOEXEC, which is
not thread safe.

This is particularly important for the "pipe" output plugin (and
others, such as JACK/PulseAudio), because we were heavily leaking file
descriptors to child processes.

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

Added an assertion in mixer_set_volume().  Removed the range checks
from the ALSA and OSS plugins.

Detect misconfiguration during MPD startup.

The MPD core guarantees that methods are always invoked in a
consistent state.  This means we can remove lots of checks from the
volume methods.

strncasecmp() is locale dependent, but we only need ASCII here.

Instead of replacing NULL with the default path in the open() method,
pass the default path to config_get_block_string().

Use config_get_block_string() instead of config_dup_block_string().

When getting or setting the volume fails, the MPD core close the
mixer.  Moved the duplicated code from the mixer plugins.

Remember if a mixer object is open or closed.  Don't call open() again
if it is already open.  This guarantees that the mixer plugin is
always called in a consistent state, and we will be able to remove
lots of checks from the implementations.

To support mixers which are automatically opened even if the audio
output is still closed (to set the volume before playback starts),
this patch also adds the "global" flag to the mixer_plugin struct.
Both ALSA and OSS set this flag, while PULSE does not.