When the decoder thread has a pending command, send the STOP command
to cancel this command.  Send STOP again if the decoder thread is
still running after that, just in case the decoder thread has executed
the previous command (which was overwritten).

dc.pipe must be non-NULL while the decoder thread is running.  Ensure
that with a load of assertions.

When the decoder initialization has not been completed yet, all calls
to dc_seek() will fail, because dc.seekable is not initialized yet.
Wait for the decoder to complete its initialization, i.e. until it has
called decoder_initialized().

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.

decoder_control.thread contains the handle of the decoder thread, or
NULL if the decoder thread isn't running.

When MPD exits, it should manually free all resources in use, to allow
easy memory leak debugging.  Make the decoder thread terminate during
that.

The player did not care about the exact error value, it only checked
whether an error has occured.  This could fit well into
decoder_control.state - introduce a new state "DECODE_STATE_ERROR".

Renamed variables and functions.

Don't return 0/-1 on success/error, but true/false.  Instead of int,
use bool for storing flags.

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

Typedefs shouldn't be used, use the bare struct names instead.

Again, a data type which can be forward-declared.

Destroy the mutex when it is not used anymore.

Now that "dc" is available here, we don't have to pass it to
decoder_is_idle() and decoder_is_starting() anymore.

The source "decoder_control.c" provides an API for controlling the
decoder.  This replaces various direct accesses to the DecoderControl
struct.