demux: don't use codec_tag for raw PCM formats

In commit 2dd2d9b, raw PCM was switched to always go through ad_lavc,
and mapping codec IDs to mplayer internal codec tags was removed, as it
was not needed anymore. However, some uncompressed audio lavf demuxers
export  their own codec tags, which collide with the existing internal
mplayer codec tags, leading to incorrect raw PCM codec selection based
on the misinterpreted audio tag. Re-add the mapped codec IDs from
2dd2d9b. Map them to an invalid codec tag, so that the generic lavc
decoder is selected (assumes ad_lavc is the decoder for raw PCM).
demux_mkv: support more formats with V_UNCOMPRESSED

Select the generic raw video decoder in codecs.cfg ("MPrv" FourCC),
which forces the generic lavc raw video decoder "rawvideo". This means
all FourCCs understood by lavc rawvideo are supported, not just whatever
has codecs.cfg entries.
mplayer: add newline for --print-msg
screenshot: minor simplification, prefer VF over VO

Remove screenshot_force and associated logic. Always try to use the
screenshot video filter before trying taking screenshots with the VO,
which means that --vf=screenshot now takes the role of --vf=screenshot_force.
(To make this clear, not adding a video filter is still the recommended
way to take screenshots; we just change how VF screenshots are forced.)

Preferring VO over VF and having --vf=screenshot_force used to make
sense when not all VOs supported screenshots, and some VOs had somewhat
broken screenshots (like vo_xv taking screenshots with OSD in it). But
all these issues are fixed now, so just get rid of the cruft.