Add pa_channels_valid()

I think this makes the code a bit nicer to read and write. This also
reduces the chances of off-by-one errors when checking the bounds of
channel count values.

src/map-file

@@ -25,6 +25,7 @@ pa_channel_map_valid;
 pa_channel_position_from_string;
 pa_channel_position_to_pretty_string;
 pa_channel_position_to_string;
+pa_channels_valid;
 pa_context_add_autoload;
 pa_context_connect;
 pa_context_disconnect;