Add -c/--channels option to enable multichannel output

Default value is 2.

This removes CHANNELS macro definition. Some statically defined arrays
now are defined dynamically based on the number of channels specified.

* output_ports: Initialized by jack_init(), freed by jack_shutdown().
* delays: Initialied by audio_init(), freed by audio_close(), which is
  called by atexit() on program exit.
* sound.vcf, sound.formant_history: Initialized by new_sound(), freed
  also by new_sound() just before replacing old sound in `sounds'
  array, and by audio_close().
Fix memory leaks related to sndfile and sample data

* free sndfile after reading it
* free sample data array when fixing rate
* free list of output ports before reusing variable
  (only if INPUT is defined)