Found with readability-const-return-type
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This is just the API design; there is no implementation yet, and no
caller.

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

Coverity discovered that the Pulse plugin could throw exceptions from
Pause(), but that method was marked "noexcept" because its caller was
not designed to catch exceptions.  So instead of avoiding exceptions
(by catching and logging them in each and every implementation), let's
allow them, and do the catch/log game in the MPD core.

Fixes regression from commit 31bad5f7:
if Pause() returns true, the output thread is running a busy loop,
causing 100% CPU usage, instead of just closing the output.

Closes #105

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Use milliseconds precision to reduce rounding errors.

Eliminates explicit global initialisation.

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.

"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.

The tag pool is a shared global resource that is infrequently
modified.  However, it can occasionally be modified by several
threads, especially by the metadata_pipe for streaming metadata
(both reading/writing).

The bulk tag_item pool is NOT locked as currently only the
update thread uses it.

Don't call tag_pool_get_item() for duplicating tags, just increase the
item's reference counter instead.

The new source tag_pool.c manages a pool of reference counted tag_item
objects.  This is used to merge tag items of the same type and value,
saving lots of memory.  Formerly, only the value itself was pooled,
wasting memory for all the pointers and tag_item structs.

The following results were measured with massif.  Started MPD on
amd64, typed "mpc", no song being played.  My music database contains
35k tagged songs.  The results are what massif reports as "peak".

 0.13.2:     total 14,131,392; useful 11,408,972; extra 2,722,420
 eric:       total 18,370,696; useful 15,648,182; extra 2,722,514
 mk f34f694e: total 15,833,952; useful 13,111,470; extra 2,722,482
 mk now:     total 12,837,632; useful 10,626,383; extra 2,211,249

This patch set saves 20% memory, and does a good job in reducing heap
fragmentation.

This flag is used internally; it is set by decoder_seek_where(), and
indicates that the decoder plugin has begun the seek process.  It is
used for the case that the decoder plugin has to read data during the
seek process.  Before this patch, that was impossible, because
decoder_read() would refuse to read data unless dc->command is NONE.
This patch is kind of a dirty workaround, and needs to be redesigned
later.

"decoder plugin" is a better name than "input plugin", since the
plugin does not actually do the input - InputStream does.  Also don't
use typedef, so we can forward-declare it if required.