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.

For spotify playlists or tracks. Uses a spt uri, so with mpc you can
add playlists with

  mpc load spt://spotify:user:simon.kagstrom:playlist:3SUwkOe5VbVHysZcidEZtH

Replaced all occurrences of g_error() with MPD_ERROR() located in a new header
file 'mpd_error.h'. This macro uses g_critical() to print the error message
and then exits gracefully in contrast to g_error() which would internally call
abort() to produce a core dump.

The macro name is distinctive and allows to find all places with dubious error
handling. The long-term goal is to get rid of MPD_ERROR() altogether. To
facilitate the eventual removal of this macro it was added in a new header
file rather than to an existing header file.

This fixes #2995 and #3007.

If the MIME type of a stream contains a semicolon, remove it and the
text after it to probe for a playlist plugin.

Merged both loops into playlist_list_open_stream().  This is needed
because playlist_list_open_stream() needs to know the MIME type, which
is only known after the stream has become "ready".

Fixes an assertion failure in the input_stream_seek() call.

Memory leak fix.  The input_stream object passed to
playlist_list_open_stream_suffix() must be closed by the caller - this
however never happens in playlist_list_open_path(), because it does
not return it to the caller.

This playlist plugin handles FLAC files with embedded CUE sheets.

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).

Without this, seeking may cause MPD to crash.

This plugin is the groundwork for MPD's future generic CUE sheet
support.  That's not complete yet, e.g. there is no way for a playlist
plugin to address an arbitrary position within a music file.

When no plugin matches the URI scheme, try the file name suffix.

Use open_uri() / open_stream() only after checking that they are
implemented.

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.

Drop the required GLib version from 2.16 to 2.12, because many current
systems still don't have GLib 2.16.  This requires several new
compatibility functions in glib_compat.h.

No CamelCase.  Use bool instead of int.  Make both arguments
mandatory.

Added an interface for loading playlists from a local file.

This new plugin parses extm3u files.  Files without the "#EXTM3U"
header are still parsed by the plain old "m3u" plugin.

If one plugin has failed to open the playlist, it may have consumed a
part of the stream already.  This may lead to a failure in all
following plugins.  Fix: rewind the stream before each open() call.

Based on the XSPF playlist plugin.

This plugin will replace the last.fm input plugin, once the playlist
API is integrated into MPD.

This patch completes the configuration support.

Prevent access on uninitialized variable if the plugin list is empty.

Based on this API, we will add parsers for EXTM3U, PLS, ASX, last.fm
radio and others.

There is no integration into the MPD core yet.  Right now, we have a
command line test program.  This is work in progress.