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.