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.