In order to be able to apply equlization at once on the mouth output and
the speaker input, two MSEqualizer filters are instancited at audiostream
starting.

The old 'eq_active' and 'eq_gains' linphonerc parameters have been disabled
as they are no more sufficient to set different parameters on the speaker and
microphone equalizers. Those have been replaced by four parameters that
explicitly specify the equalizer to set up.

These new parameters are 'mic_eq_active' and 'mic_eq_gains' for the microphone equalizer and
'spk_eq_active' and 'spk_eq_gains' for the speaker equalizer.

An extra parameter has also been added in audio_stream_enable_equalizer() and
audio_stream_equalizer_set_gain() in order to indicate which equalizer is
concerned by the modification.