ffmpeg: Ignore SIGPIPE

ffmpeg: Ignore SIGPIPE

On systems which deliver SIGPIPE (Unices), a broken pipe will currently
result in the immediate termination of the ffmpeg process (the default
disposition as required by POSIX).  This is undesirable, because while
the broken pipe is likely fatal to useful cleanup of whatever component
is writing to it, there might be other components which can do useful
cleanup - for example, a muxer on another stream may still need to write
indexes to complete a file.  Therefore, set the signal disposition for
SIGPIPE to ignore the signal - the call which caused the signal will
fail with EPIPE and the error will be propagated upwards like any other
I/O failure on a single stream.