lavc: clear AVBuffers on decoded frames if refcounted_frames is not set

Otherwise some code might mistakenly think it is allowed to unref those
buffers, which would lead to double unref.
vf_split: fix description

It now allows an arbitrary number of inputs, not just two.
af_channelmap: remove now unnecessary goto

Options are freed from the generic code now, there is no need to call
av_opt_free() from the filter.
af_channelmap: ensure the output channel layout is valid.
graphparser: only print filter arguments if they are non-NULL
vf_setpts: cosmetics, reformat
lavfi: add an asetpts filter
lavfi: add trim and atrim filters.
avconv_opt: allocate an OutputFile before the streams.

This way OutputFile variables like recording time can be used when
creating the streams.
avconv: make -t insert trim/atrim filters.

This makes -t sample-accurate for audio and will allow further
simplication in the future.

Most of the FATE changes are due to audio now being sample accurate. In
some cases a video frame was incorrectly passed with the old code, while
its was over the limit.
avconv: make output -ss insert trim/atrim filters.

This makes output -ss sample-accurate for audio and will allow further
simplication in the future.