pactl, pacmd: Allow to unset the configured default sink or source

Currently there is no way to unset the default sink or source once it was configured manually by the user. This patch introduces the special name @NONE@, which can be used with the pacmd or pactl set-default-sink and set-default-source commands to unset the user configured default. When the default is unset, pulseaudio will return to the standard default sink or source selection mechanism based on priority.