Fixes build failure on WIN32.

This fixes the following valgrind warning occuring on the first call of
httpd_output_read_page:
==20124== Conditional jump or move depends on uninitialised value(s)
==20124==    at 0x425E65: httpd_output_read_page (httpd_output_plugin.c:240)
==20124==    by 0x426087: httpd_output_open (httpd_output_plugin.c:279)
==20124==    by 0x41D862: ao_open (output_plugin.h:206)
==20124==    by 0x41E133: audio_output_task (output_thread.c:590)

This cleanup call is obsolete, since we moved the binding code to
enable()/disable().

RFC 5334 10.3 defines the MIME type "audio/ogg".  We could use
"application/ogg" as well, but we know for sure that we only emit
audio data.

Added support for a new optional configuration setting for the httpd output
named "bind_to_address". Setting it to a specific IP address (v4 or v6) will
cause the httpd output to bind to that address exclusively. Supporting
multiple addresses in parallel is future work.

This implements the feature requests #2998 and #2646.

Work around aliasing warning.

Send silence to all connected clients while paused, to avoid
connection interruption.

Avoid buffer underruns on the streaming client, if the encoder is "too
efficient" (e.g. when encoding silence while paused).

libwrap support is in MPD but only for the control port.  This patch
adds support for the http port.  The code is copied from
src/client_new.c

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.

Changed the wrappers for pipe(), socket(), accept().  On WIN32, this
does not work for pipe().

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.

Implement the methods enable() and disable().  Bind the HTTP port in
the enable() method, but reject all incoming connections until the
output is opened.

The first patch by Patrick didn't work, because his "#ifdef HAVE_OSX"
line would have required config.h.

On Mac OS X, the httpd plugin cannot be compiled, because OS X's
system headers do nto include sys/types.h, although they use
u_int32_t.

Hello,

While compiling latest git I've received a compile error in the
httpd_output_plugin.
Small patch attached.

Patrik

Flush the encoder before calling encoder_tag().  The first page
generated by the encoder after sending the tag will be the new
"header" page, which is sent to all HTTP clients when they connect.
This is a little bit specific to the vorbis encoder, but there are no
other encoders which support tags (yet).

Moved some code from httpd_output_encode_and_play() into separate
functions httpd_output_broadcast_page() and
httpd_output_encoder_to_clients().

There's no reason to send both encoder tags and Icy-Metadata to the
client.  Let's disable Icy-Metadata when the encoder supports embedded
tags.

In the tag() method, MPD guarantees that it does not pass tag==NULL.
Converted the runtime check to an assertion.

[mk: folded with patch "Put icy related functions in extra source
files"; moved icy_server.c from HAVE_CURL to ENABLE_HTTPD_OUTPUT;
removed an unused variable]

The httpd_client_check_queue() callback function does not use its
"user_data" argument.  Don't pass any, and fix the gcc warning.

Check if encoder_plugin!=NULL, not encoder_plugin_get (which is a
function).

Let's get rid of the "shout" plugin, and the awfully complicated
icecast daemon setup!  MPD can do better if it's doing the HTTP server
stuff on its own.  This new plugin has several advantages:

- easier to set up - only one daemon, no password settings, no mount
  settings
- MPD controls the encoder and thus already knows the packet
  boundaries - icecast has to parse them
- MPD doesn't bother to encode data while nobody is listening

This implementation is very experimental (no header parsing, ignores
request URI, no icy-metadata, ...).  It should be able to suport
several encoders in parallel in the future (with different bit rates,
different codec, ...), to make MPD the perfect streaming server.  Once
MPD gets multi-player support, we can even mount several different
radio stations on one server.