Add transcoding via filter scripts to http streaming

This adds a new parameter to the http streaming server.
Adding ?transcoder=test makes it pipe all http output through a "filter", which can do basicaly anything with it.

It searches for the trascoders in hts_settings_get_root()/transcoders
It also passess all GET parameters in hc_req_args as environment variables, prefixed with GET_, so the transcoder can get options via url parameters.

This also modifies the writing to the socket a bit, so it checks if it can write to the http stream, because it can no longer rely on write() errors, because it no longer directly writes to the tcp socket. I use select and poll for this.

There is still a problem somewhere, because in my tests a transcoding vlc throws a lot of warnings about broken input data. Connecting with vlc via http, which should bring the same data to vlc, works perfectly. But this only seems to happen with higher bandwith streams, so i guess it's some buffer overflowing.

But in general, this already works fine.