When closing files that were opened for writing, cached data will not
be flushed to storage automatically anymore. This reverts the
workaround implemented for #2469. A new option was added to both
programs (`--flush-on-close`) that re-enables flushing for people who
are affected by data loss such as described in #2469.

The reason is that automatic flushing causes long delays in processing
queues when the output by mkvmerge/mkvextract isn't the final product but
just an intermediate result to be processed further.

Implements #2480.