Prepare the transition to std::mutex.

implement for the "load" command the same logic used for the "add"
command: local clients can load playlist specified as absolute paths.

For relative paths the old logic is preserved: first look for a stored
playlist, then look in the music directory.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Simplifies a lot of code, because we don't need to return both the
SongEnumerator and the InputStream.

Allows forward-declaration.

Replaces struct playlist_provider.

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.

The new function playlist_open_any() combines playlist_mapper_open(),
playlist_list_open_uri() and playlist_list_open_stream(), providing an
easy API for all of them.

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 updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

The new generic encoder API will replace shout's custom encoder API.

This patch implements the MMS protocol, by using libmms.  It is quite
experimental: it does not support seeking yet, and it is currently
using synchronous I/O, which causes MPD to hang while waiting for the
server.

In generic PCM functions allowing all sample formats, pass a void
pointer instead of a char pointer.

Moved the software mixing code (used by crossfading) to a separate
library.

Don't use libc's rand() function, because it is slow.  Our own trivial
linear congruential generator is good enough for dithering.

redirect_stdin() is a daemonization function, and disconnecting from
the standard input is always a good idea for MPD.

Prepare for the migration to the GLib option parser, which uses
gboolean for flags.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

When there are standardized headers, use these instead of the bloated
os_compat.h.