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.

To check for errors without reading.  The decoder thread wants to do
that, before it passes the input stream to the plugin.

Reduce heap usage by reducing the number of malloc() / free() calls.

Update the struct attributes, important for facades like the "rewind"
plugin.  To replace buffer().

The assertion added in MPD 0.15.14 was too much, it failed when the
MIME type of a stream was NULL.

Duplicate the "mime" attribute of the inner input_stream object,
instead of copying the pointer.

The assumption that MIME type is set only once is not valid with CURL,
as URL redirections may update the MIME type.

This fixes bug #3044.

All close() implementations must call this method.

Don't limit the "rewind" input plugin to CURL streams.

Major API redesign: don't let the caller allocate the input_stream
object.  Let each input plugin allocate its own (derived/extended)
input_stream pointer.  The "data" attribute can now be removed, and
all input plugins simply cast the input_stream pointer to their own
structure (with an "struct input_stream base" as the first attribute).

This replaces the rewinding buffer code from the CURL input plugin.
It is more generic, and allows rewinding even when the server sends
Icy-Metadata (which would have been too difficult to implement within
the CURL plugin).

This is a rather complex patch for the stable branch (v0.15.x), but it
fixes a serious problem: the "vorbis" decoder plugin was unable to
play streams with Icy-Metadata, because it couldn't rewind the stream
after detecting the codec (Vorbis vs. FLAC).