This was done with the help of callcatcher [1]. Only functions which
are statically known to be unused are removed.
Some unused functions are not removed yet, because they might be needed
in the near future (such as open_output_stream for the encode branch).
There is one user visible change: the --subcc option did nothing, and is
removed with this commit.
[1] http://www.skynet.ie/~caolan/Packages/callcatcher.html
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