Eliminate input_stream_init() and input_stream_deinit().

This is the version present in Ubuntu Oneiric, the oldest distribution
with gcc 4.6.  Debian Squeeze is off target, because it has gcc 4.4,
which is unable to compile MPD anyway.

This commit drops all API compatibility hacks for older versions.

Hide the definition from C code, to prepare the transition to C++.

This is part of libavutil.

URLContext is deprecated.

Support ancient ffmpeg versions.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

FFmpeg/libav have dropped AVIO_RDONLY in favor of AVIO_FLAG_READ. This
patch fixes that in MPD.

URLContext is deprecated.

All modern distros ship 0.5, so there's no need to support old and buggy
ffmpeg versions.