lavfi/ebur128: remove now unused av_opt_free call.
lavfi/hue: apply major simplifications, and switch to AVOption-based system

This also drops support for "flat syntax" and "reinit" command.

"reinit" command is not very robust and complicates the logic more than
necessary, since requires to reset all the options in the command.

*This is a syntax break*.
lavfi/hue: reindent after last commit
lavfi/hue: extend debugging message

In particular, show h value in degrees.
lavfi/flite: switch to AVOption-based system