Make volume component work on FreeBSD

- Edit compile instructions
- Reuse OpenBSD sndio implementation

config.def.h

@@ -59,7 +59,7 @@ static const char unknown_str[] = "n/a";
  * uptime              system uptime                   NULL
  * username            username of current user        NULL
  * vol_perc            OSS/ALSA volume in percent      mixer file (/dev/mixer)
- *                                                     NULL on OpenBSD
+ *                                                     NULL on OpenBSD/FreeBSD
  * wifi_perc           WiFi signal in percent          interface name (wlan0)
  * wifi_essid          WiFi ESSID                      interface name (wlan0)
  */