avfilter: avoid direct access to "frame"->channels

This avoids ABI issues

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavc/utils: fix typo and reword message in case of too low bitrate
lavfi/unsharp: add missing option flags
cmdutils: fix stylenits
cmdutils: make print_all_libs_info() show libavresample version

Even if not directly supported by FFmpeg, it is still useful to see the
linked version of libavresample.
cmdutils: remove duplicated check on CONFIG_POSTPROC

The check is already performed by the PRINT_LIB_INFO macro.
cmdutils: clarify log message, in case of option routed to both codec and mux/demux layer
doc/ffplay,ffmpeg: mention the corresponding negated option for -stats

This helps lazy users who didn't read the part about boolean options, in
case they grep for "-nostats".

-stdin and -stats are the only booleans option enabled by default. This
is a partial fix for Trac ticket #2332, a complete overhaul of boolean
options documentation is probably overkill.
ffmpeg: port sub2video to AVFrame.

And re-enable the FATE test.
ffmpeg: add the -canvas_size option.

Allows, amongst other things, to override the size guessed
by the sub2video hack.

Note: the -s option could have more or less the same semantic,
but it receives a special treatment by the options system.
lavfi/buffersrc: implement flags.

The PUSH flags is necessary for efficient scheduling;
otherwise there is no feedback when adding frames to
closed paths.

The NO_CHECK_FORMAT is a small optimization that does
not cost much to implement.

The KEEP_REF flag maps to the add/write distinction in
the fork's API.
ffmpeg: re-enable use of buffersrc flags.

AV_BUFFERSRC_FLAG_PUSH is necessary to detect closed streams.
lavfi/smartblur: add support to named options
lavfi/sink_buffer: move stuff to reduce the diff.

Note: av_buffersink_get_samples() is not yet implemented,
abuffersink is not yet a drop-in replacement of the fork's
abuffersink.
lavfi: remove buffersink.c.

It only contains merge conflicts leftovers.
The real code is in sink_buffer.c.
lavfi: rename sink_buffer.c into buffersink.c.
lavfi/buffersrc: do not dereference NULL frame.

frame = NULL is used to indicate EOF.
lavfi/buffersink: implement av_buffersink_get_samples().

Note: the implementation could be more efficient, but at
the cost of more diff.

Most of the code from the following commit:

commit a2cd9be212fca02dd3d6ee65cb6ab9f84c5e28e5
Author: Anton Khirnov <anton@khirnov.net>
Date:   Fri May 4 19:22:38 2012 +0200

    lavfi: add an audio buffer sink.

Adapted to call av_buffersink_get_frame_flags() instead of
accessing the frame directly.
cmdutils: remove conditionnal pp include.

The definition of the include are needed for builds even without pp.
This should fix FATE with --disable-gpl after 63edd2f9.
lavfi/buffersink: move the filter structures at the end.

Also apply a few cosmetic fixes (spaces, trailing comma) to
help match them with the fork's implementation.