sndio: Add module.

- can control sndio: change volume, toggle mute
- appearance is somewhat dynamic: takes muted status into account
- uses polling inside sleeper thread to update values
- uses sioctl_* functions, requires sndio>=1.7.0.

README.md

@@ -67,6 +67,7 @@ libnl [Network module]
 libappindicator-gtk3 [Tray module]
 libdbusmenu-gtk3 [Tray module]
 libmpdclient [MPD module]
+libsndio [sndio module]
 ```
 
 **Build dependencies**