avfilter/formats: allow unknown channel layouts by default

Since the default in the libav fork is to only allow known layouts, making
unknown layouts allowed by default here can be a security risk for filters
directly merged from libav. However, usually it is simple to detect such cases,
use of av_get_channel_layout_nb_channels is a good indicator, so I suggest we
change this regardless.

See http://ffmpeg.org/pipermail/ffmpeg-devel/2016-November/203204.html.

This patch indirectly adds unknown channel layout support for filters where
query_formats is not specified:

abench
afifo
ainterleave
anullsink
apad
aperms
arealtime
aselect
asendcmd
asetnsamples
asetpts
asettb
ashowinfo
azmq

It introduces a query_formats callback for the asyncts filter, which only
supports known channel layouts since it is using libavresample.

And it removes .query_formats callback from filters where it was only there to
support unknown layouts, as this is now the default:

aloop
ametadata
anull
asidedata
asplit
atrim

Acked-by: Nicolas George <george@nsup.org>
Signed-off-by: Marton Balint <cus@passwd.hu>
