Various minor fixes

In this pull request I collected the following fixes:

1. sound: Fix port handling for the unknown availability case
  It's required for proper support for pulseaudio 3.0.

2. Fix icons on Sound preferences dialog
  It's a renaming problem, the icons have a prefix.

3. Fix faces location on User accounts dialog
  Also a renaming problem. The /usr/share/pixmaps/faces location is used by gnome-control-center. Cinnamon uses /usr/share/pixmaps/cinnamon/faces instead.

4. Use freedesktop as default sound theme instead of ubuntu
  Since the freedesktop is the default sound theme in gsettings-desktop-schemas, it should be also in cinnamon-control-center, otherwise it crashes.