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

Replaces GLib's GError.

This is necessary for Ogg packets that span more than one page.

We can only check for negative values if the variable is signed.

Make sure the file header gets written at the beginning, before
_write() gets called.

Add the encoder_plugin method end().  This is important for the
recorder plugin.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Move the "extern" declarations from output_list.c, for more type
safety.

Windows compatibility.

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.

The recorder plugin writes audio played by MPD to a file.  This may be
useful for recording radio streams.

This implementation is incomplete, because support for tags is
missing, and MPD should be able to record each track to a different
file.