Partially undoes commit 2c4e08d8:

    riff: always generate a proper WAVEFORMATEX structure in
    ff_put_wav_header

A new flag, FF_PUT_WAV_HEADER_FORCE_WAVEFORMATEX, is added to force the
use of WAVEFORMATEX rather than PCMWAVEFORMAT even for PCM codecs.

This flag is used in the Matroska muxer (the cause of the original
change) and in the ASF muxer, because the specifications for
these formats indicate explicitly that WAVEFORMATEX should be used.

Muxers for other formats will return to the original behavior of writing
PCMWAVEFORMAT when writing a header for raw PCM.

In particular, this causes raw PCM in WAV to generate the canonical
44-byte header expected by some tools.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>