Major change of the channels management. p_format->i_channels disappeares

Major change of the channels management. p_format->i_channels disappeares
and is replaced by two members : i_physical_channels and i_original_channels.
i_physical_channels describes the channels which are effectively present
in the stream (at the current point of the aout pipeline), whereas
i_original_channels represents the channels we used to constitute the
stream. For instance a mono stream (FRONT_CENTER) may emanate from a
FRONT_LEFT channel of a STEREO stream.

Additionally, this contains hooks to dynamically select the audio device
and the channels configuration. In order to do that, all aout plugins,
and all interface plug-ins need to be adapted. Currently only SDL
(partially OSS) and the rc interface have been adapted, please use them
as a guide. Other aout plug-ins have been temporarily disabled.

IMPORTANT : if you need a working sound output, DO NOT UPDATE YOUR TREE
for several days until we fix that.

* modules/misc/network/ipv4.c: Fixed a long-standing segfault when saving
  preferences and asking for multicast.